cleanup: #642 follow-ups — aac literal, stale doc, dead any_of, test-only is_borrowed

[crippledgeek]

Follow-up to #642 (closed via #647). Small, independent cleanups surfaced by the code review; grouped because each is a few lines.

• "aac" tier-3 fallback is a literal at 3 sites in audio_encoder_registry.rs (:87, :499, :633). It encodes the "when all else fails" policy, which is currently invisible unless you read the line. Wants one named const carrying the rationale.
• Stale doc comment — rdlp-types/src/postprocess.rs:176 still describes video_encoder = "" as meaning "no override". refactor: replace stringly-typed codec/encoder names with validated newtypes (#642) #647 made that value rejected, so the doc now asserts a guarantee the code does not provide.
• any_of is #[allow(dead_code)] (recode.rs) — the general OR-combinator was added for completeness but no rule_for arm uses it. Per house rule, no dead code kept for future use: exercise it or drop it.
• is_borrowed is public but test-only (media_name/mod.rs) — its only callers are tests/media_name.rs. Either #[cfg(test)] pub(crate) it, or keep it as the executable form of the allocation-free contract and say so in the doc.