fix(a11y): preserve line breaks in imeta alt text, capped at one blank line

[dmnyc]

Problem

The ALT editor is a multiline textarea — we invite multi-paragraph descriptions — but every publish path flattened the alt slot's \n to a space. Authored paragraphs were silently destroyed at publish, for every reader including our own (verified against a real published note, id 3f66caf7…).

Fix

• normalizeAltBreaks (new, src/lib/feed/imeta.ts): CRLF → LF, trims each line, caps runs of 3+ newlines at exactly \n\n (one blank-line paragraph gap), trims the ends.
• Publish paths preserve line breaks instead of flattening: buildImetaTag (notes, replies, recipes, forks, longform covers, products) and withImetaAlt (edit/fork tag round-trip).
• Read paths cap (parseImeta, imetaAltByUrl): alt received from any client can't balloon the layout.
• Display: the fullscreen lightbox caption gets white-space: pre-line (the Description dialog already used pre-wrap).

Wire format example

NIP-92 slots are "key + everything after the first space", so a real \n inside the tag string survives the round trip: JSON escapes it on the wire, relays store it verbatim, event ids/signatures are unaffected. Built with the shipping code and parsed back byte-identical:

[
  "imeta",
  "url https://i.nostr.build/eS5EHNjnkLY9cJ4iQ47pOO.png",
  "alt A screenshot of a Nostr post about web accessibility.\n\nBelow it, a quoted post from @BlindBitcoiner: \"Hello world this is a test post. I'm totally blind and trying Damus to see if blind users can use this Nostr client.\""
]

Pre-fix, the same input published as a single flattened line.

Interop note

NIP-92 is silent on newlines; this adopts the no-invention option (raw newlines, no escape scheme). Clients parsing slots on the first space get the full multi-paragraph text; clients splitting on any whitespace see only the first line — a degraded but valid description, versus today's loss for everyone.

Mobile parity

docs/accessibility/alt-text-imeta-linebreaks-mobile-handoff.md is an agent-facing spec for the Android/iOS apps: normative rules, reference normalization in TS/Kotlin/Swift sketches, parse requirements (first-space split), display/authoring rules, and test vectors. §1 of the main handoff doc (alt-text-imeta-handoff.md) is updated to the new wire contract.

Tests

Flatten assertions replaced with preserve/cap/CRLF/JSON-round-trip/received-cap vectors (38 imeta tests) ; the Amethyst interop vector is unchanged and passing. noteReview suite green (71).