canary: add long-form chunking

[brauliobo]

Dependency

This is a stacked PR on top of #111 (canary: add v2 word timestamps). The branch intentionally includes that commit; please review this PR after #111, then rebase it onto main once #111 lands.

Summary

• add offline long-form transcription for Canary 1B v2 using 30-40 second windows, low-energy acoustic boundaries, and 1 second of overlap
• rebase timestamped chunk output onto the source timeline and assign overlap words by midpoint ownership
• reconcile text-only overlap against the previous hypothesis suffix so repeated phrases are not mistaken for duplicated context
• report Canary 1B v2 audio capacity as unbounded while leaving older Canary variants at their existing positional-table limit
• document that long-form input remains memory-resident and batch rows are processed serially

Motivation

Canary 1B v2 has a 40 second decoder window. Running longer audio as one window can silently omit substantial interior speech even when inference reaches the final phrase. On the 84.381 second samples/whole-earth.wav, the pre-long-form path returned 141 normalized words with a 35.040 second internal transcript gap. The chunked path returned 217 normalized words with a maximum 1.840 second inter-word gap and continuous coverage through the final phrase.

The first fixed-duration prototype also lost repeated words at chunk seams. Selecting low-energy boundaries and restricting text deduplication to a true suffix fixed that failure rather than masking it with a broader overlap heuristic.

Correctness

Adversarial repeated-speech tests passed without missing or duplicated repetitions:

Audio	Words	Result
44 s	88	exact 4x repetition
55 s	110	exact 5x repetition
88 s	176	exact 8x repetition
121 s	242	exact 11x repetition

The 39 s, 40 s, 40.001 s, and 41 s threshold cases also passed. Every benchmark returned monotonic positive word intervals within the audio duration, with normalized full text matching the word sequence.

Performance

Warm Q8_0 measurements on a shared RTX 4060 Ti:

Mode	55 s	121 s
timestamps	2,784.51 ms (19.8x realtime)	6,366.06 ms (19.0x)
text only	2,042.28 ms (26.9x realtime)	4,376.20 ms (27.6x)

F32 timestamp validation at 55 seconds passed on an RTX 3090 in 899.36 ms (61.2x realtime). The shared RTX 4060 Ti did not have enough free memory for the F32 model's 6.1 GiB CUDA allocation, so that cell was moved to the 3090 rather than changing running services.

Verification

• 52/52 configured CTest tests passed; 18 unrelated fixture-gated tests remained skipped by default
• gated timestamped and text-only Canary Q8_0 real-model smoke passed manually
• pinned clang-format check passed
• git diff --check passed