fix(u7): six defects from the heid bug-hunt panel, and five vacuous falsifiers

Cross-frontier panel (Gróa/Hulda/Regin/Kimi) on U7's diff, thread
01M368G2Y0JMTJ2T7M3JMTXV5Z. Four of the six fixes are for defects no test in
this repo could have caught, and the panel's guard-strength passes found five of
my own falsifiers green under the exact change they forbade.

THE 4-OF-4 FINDING — the group anchor could land on the WRONG artifact.
The anchor was the raw rel spliced into an href fragment while the tile id was
equally raw. A browser matches a fragment against ids RAW FIRST and only then
percent-decoded, so raw-on-both-sides is not merely unencoded, it is AMBIGUOUS:
with `a b.png` and `a%20b.png` in one booth, the first's href resolves to the
fragment `item-a%20b.png` and the raw pass matches the SECOND file's id. That is
the misfiled-judgment failure invariant 6 exists to prevent, arriving through a
path invariant 6 never looked at. Both sides now use `Item.url`
(`quote(rel, safe="/")`), which is injective here and is the convention
booth_flag has always used. The original test asserted the href occurred as SOME
id on the page — true while pointing at the wrong one.

GRÓA'S STRONGEST SOLO — a zero-hit filter removed the way back.
The rail was gated on the FILTERED list, so a valid filter with no matches
removed the rail, the filter links and the route back to `all`, while the
empty-booth branch announced the booth was empty with rail.total still holding
the real count. No recovery without editing the address bar, and it degraded the
same way with JavaScript off, on the surface the operator actually reviews on.
Gated on all_items now, with an explicit no-match row.

HULDA — one unrepresentable filename took out the INDEX, not just its booth.
A non-UTF-8 filename reaches CPython as a surrogate and quote() raises on it,
outside any per-item handler. booth_items feeds list_booths, so one 0xff byte in
one booth's filename 500s every booth's card. Such a file cannot be linked,
served or zipped, so it is skipped like a dotfile.

HULDA — the `f` shortcut has never worked. The selector named `.flagbtn`, which
nothing in this repo emits, so it fell through to the hidden target input;
clicking a hidden input does not submit its form, and the handler called
preventDefault anyway. Now clicks the flag form's real button, verified end to
end in a real browser.

GRÓA — a group jump was undone by the next keypress. The jump scrolls, the
cursor stayed at -1, and the next arrow focused tile 0 and scrolled back. The
cursor now picks up from the viewport, which also fixes the general
scroll-then-arrow case. Asserted on real scroll geometry in Chromium.

HULDA — the caption sidecar was read whole before being truncated, so a
pathological file was a MemoryError the OSError handler does not catch. Bounded
at the read, and deliberately NOT by st_size: a FIFO reports 0.

ACCEPTED KNOWN RISKS, both now documented rather than implied: no cap on rail
row count (1,000 groups of two would render 1,000 rows; the largest live booth
is 66 items and picking a cap without a booth that needs one is invented work),
and Item.group sits mid-dataclass (one construction site, keyword-only, grepped).
The docstring now names the UPPER median explicitly — two arms flagged that
"the middle group" admits both readings for an even count.

FIVE VACUOUS FALSIFIERS, found by the arms and not by me: the anchor test
survived v[0]->v[-1]; the informativeness guard survived sizes[-1]; the group
count survived len(v)+1; the zero-hit filter test used a fixture that HAD hits;
and the escaping test asserted over the whole page, so it went red on a code
comment. All rewritten, all mutation-proved. The table is up to 20 rows and one
drifted when I changed the line under it — reported by the harness, not silently
skipped, which is the behaviour tests/test_mutation_check.py exists to hold.

660 green; 20/20 proved. Deployed; 21/21 booths 200.

Held for design-dev, not fixed here: Gróa's finding that the sticky rail has no
scroll-margin, so a fragment jump tucks the target under it. It is one line in
base.html, the file he is rewriting from scratch.

tests/test_items.py

@@ -13,6 +13,7 @@ from fastapi.testclient import TestClient
 
 from booth.app import build_gallery, create_app, list_booths
 from booth.items import (
+    CAPTION_MAX,
     Item,
     booth_items,
     find_item,
@@ -316,3 +317,42 @@ def test_booth_items_carries_the_group(tmp_path):
     _touch(b / "99.png")
     got = {it.rel: it.group for it in booth_items(b)}
     assert got == {"ac01.png": "ac", "ac02.png": "ac", "99.png": None}
+
+
+def test_one_unrepresentable_filename_costs_its_own_tile_not_the_booth(tmp_path):
+    """HULDA, and it is worse than the bundle could see: `quote()` raises
+    UnicodeEncodeError on a surrogate from a non-UTF-8 filename, and
+    `booth_items` feeds `list_booths` — so ONE 0xff byte in ONE booth's
+    filename took out the INDEX for every booth, not just its own page.
+
+    The repo's standing posture is that a damaged file costs its own tile and
+    never the page. A file whose name cannot be percent-encoded cannot be
+    linked or served either, so it cannot be an item.
+
+    Defeating change: dropping the guard — this raises before it renders."""
+    import os
+
+    b = tmp_path / "b"
+    b.mkdir()
+    (b / "ok.png").write_bytes(b"\x89PNG")
+    (b / os.fsdecode(b"bad\xff.png")).write_bytes(b"\x89PNG")
+
+    got = booth_items(b)
+    assert [it.rel for it in got] == ["ok.png"]
+
+
+def test_a_huge_caption_sidecar_is_not_read_whole(tmp_path):
+    """HULDA: `read_text()` pulled the entire sidecar into memory before
+    `[:CAPTION_MAX]` trimmed it, and the handler catches only OSError — so a
+    pathological sidecar is a MemoryError, not a missing caption.
+
+    Bounded at the READ. Deliberately NOT bounded by st_size: a FIFO reports
+    st_size 0 and a bound that trusts it inherits what it does not mean —
+    persistent-memory.d/2026-09-22-size-cap-opened-a-hang.md."""
+    b = tmp_path / "b"
+    b.mkdir()
+    (b / "a.png").write_bytes(b"\x89PNG")
+    (b / "a.txt").write_text("x" * (CAPTION_MAX * 50))
+
+    cap = {it.rel: it.caption for it in booth_items(b)}["a.png"]
+    assert cap is not None and len(cap) <= CAPTION_MAX