Indigo muted words and hide-reposts

The last Cobalt parity gap was feed filtering: Cobalt honours the account's saved muted words and the home timeline's hide-reposts preference (commit d4c1686, issue #106); Indigo did not. This ports the whole module.

Changes

• src/atproto/prefs.{c,h} — pure, host-testable muted-word matching, matching Cobalt's rules exactly: case-insensitive; a single alphanumeric word matches whole words only (muting "cat" does not hide "category"); a phrase or punctuated word matches as a substring; tag mutes match facets with or without the leading #; a word with no targets is content-only. indigo_prefs_filter_page compacts a session page in place and returns how many posts were dropped.
• src/atproto/prefs_wolfram.c — the Wolfram conversion, kept out of prefs.c so the host build never sees Wolfram headers. Skips expired mutes (the server keeps the records), reads hide_reposts off the home feed view, maps content/tag targets.
• src/util/timefmt.c — indigo_time_parse_rfc3339 and indigo_time_now. The parser is Howard Hinnant's days_from_civil arithmetic rather than timegm, which devkitARM lacks; it accepts only the UTC form ATProto requires (fractional seconds tolerated, numeric offsets rejected). The epoch is long long because the 3DS's long is 32 bits and a far-future expiry would not fit.
• src/atproto/session.c — ensure_prefs() fetches preferences once per sign-in via wf_agent_get_actor_prefs_typed, non-fatal on failure (feed shown unfiltered, retried next page). do_timeline filters with home=true, do_feed with home=false — hide-reposts is a home-timeline preference only. drop_agent() resets the loaded flag.
• Tests: test_prefs and parser cases in test_time_rfc3339 ported from Cobalt's suite. AGENTS.md §28, README, and CHANGELOG updated.

Verification

• make test: 2940 checks, 0 failures (was 2845).
• make warnings clean, make snapshots unchanged.
• Clean make cross-build with devkitARM: indigo.3dsx builds. Cross-build verified only, not run on hardware.

Empty-after-filter pages keep the cursor and the app auto-requests the next page, terminating at feed end — same behaviour as Cobalt. Indigo commit fb46dea.