t27f: idea factory pre-flight -- refuse an idea the font cannot draw before writing a frame #10820

Part of epic #10560. Stacked on #10816 (the factory batch), because the font this checks is glyph57 from #10792.

Boundary

specs/t27f/idea_factory.t27 only, plus its seal. No change to glyph57, particle_ds or any other spec.

Scenarios

• An idea contains a character the 5x7 font does not have, such as φ, €, { or ~. Today glyph57_of draws it as ? and the reel ships with a wrong letter, and no test notices. Competitor rule 11 from the content-factory notes ("pre-flight: every slot bound, every text fits, else the item fails before render") is the gap.
• An idea is nothing but spaces and |. Today the reel is only the address card, with no message.
• An idea is long enough that the reel runs for minutes. Nothing caps it.

Requirements

• FR-001: missing_glyphs(t) MUST count the code points of t that the font draws as ?. A real ?, a | scene cut and a space do not count. A malformed UTF-8 byte counts.
• FR-002: first_missing(t) MUST return the byte offset of the first such code point, or -1 when there is none.
• FR-003: preflight(t) MUST return PF_EMPTY when the idea has no scene, PF_GLYPH when a glyph is missing, PF_LONG when reel_frames(t) exceeds MAX_S * FPS, and PF_OK otherwise, checked in that order.
• FR-004: main MUST write no frame when preflight(IDEA) != PF_OK.

Criteria

• Tests pin each of the four results, with Latin and Russian ideas passing and with φ, €, { and a malformed byte refused.
• All five batch ideas and the sample idea pass the pre-flight.
• On the Railway lab: t27c test-report passes with 0 vacuous tests, mutants are killed, and t27b reports 0 unsupported constructs.
• A binary generated with a refused idea writes 0 bytes, and one with a good idea writes whole 6220800-byte frames.
• The seal is re-saved and verifies.

Success Criteria

• t27c test specs/t27f/idea_factory.t27 runs 5 tests and exits 0
• t27c test-report specs/t27f/idea_factory.t27 reports 0 vacuous tests and exits 0
• t27b check specs/t27f/idea_factory.t27 reports 0 unsupported constructs and exits 0
• t27c run specs/t27f/idea_factory.t27 --idea "hello world" writes 6220800 bytes and exits 0
• t27c run specs/t27f/idea_factory.t27 --idea "φ" writes 0 bytes and exits 0
• t27c run specs/t27f/idea_factory.t27 --idea "€" writes 0 bytes and exits 0
• t27c run specs/t27f/idea_factory.t27 --idea "{" writes 0 bytes and exits 0
• t27c run specs/t27f/idea_factory.t27 --idea $'\xFF' writes 0 bytes and exits 0
• t27c run specs/t27f/idea_factory.t27 --idea " " writes 0 bytes and exits 0
• t27c run specs/t27f/idea_factory.t27 --idea "|" writes 0 bytes and exits 0
• t27c seal verify specs/t27f/idea_factory.t27 exits 0
• All five batch ideas and the sample idea from the factory batch pass pre-flight (5 tests in test suite)