feat(browser): expose bounded read-only PCM observation

[tamashi095]

Expose a bounded read-only PCM observer for the app’s existing input-source spectrum. Each observer uses its own cursor and reusable scratch chunk, skips raced/stale data, and leaves every playback shared word unchanged. Existing feed counters gain the already-present fields needed by the app’s diagnostics.

Dedicated Astra medium PASS. Focused/type/package/headless checks pass; the post-copy guard mutant fails at the intended slot-reuse assertion. All six generated artifacts and PCM worklet bytes are unchanged. No new graph tap, FFT, wire layout or render work.

Related:#434; stacked on#432. App adoption remains separate.

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[tamashi095]

SDK #434 independent Astra medium review

PASS at 7e1713ec (implementation acf30599). Reviewed the frozen #434 spec and its complete bounded diff. No source/spec changes, Rust rebuild, artifact repin, or extra browser matrix.

The observer shares the existing ring validator and maintains only local cursor, scratch and metadata state. Constructor/pull/counters/close do not store/add any shared word or write shared PCM. Independent cursors begin at the currently live read index and catch up using the existing wrap domain. Candidate attempts, including invalid/stale slots, consume the fixed 1..32 budget. Scratch planes and metadata are constructed once and reused; callback data does not expose shared PCM. Close drops ring/scratch ownership and preserves only the final counter snapshot; closed pulls are zero and callback reentry is refused.

The copy admission checks cover sequence, generation/tag, frame bounds and live interval. After copying, the observer rechecks the live interval as well as sequence, seek epoch, tag and copied metadata. This specifically rejects reservation/zeroing of a consumed slot before the writer publishes its new sequence. Full seek generation is checked when the low generation tag repeats. The focused fixture covers mono/stereo and partial tails, independent readers, missed data/wrap, invalid-slot budget, seek/torn candidates, reusable identities, shared-byte preservation, closed no-op and constructor/view allocation traps. The three counter additions read their existing named wire words and correctly disclaim multiword atomicity.

Independent validation:

• node --test sdk/test/browser-pcm-evals.mjs: 8/8 PASS, /private/tmp/dx434-review-focused.log.
• bash scripts/check-sdk-types.sh: PASS, /private/tmp/dx434-review-types.log.
• bash scripts/sdk-package.sh check /private/tmp/dx-393-current-artifacts: PASS, including generated checks, 11 CLI tests, strict extracted consumer types and public observer runtime/shared-byte assertions; /private/tmp/dx434-review-package.log.
• Independent isolated discriminator: unmodified source copy passes the existing during-copy race test. Removing only the post-copy live-interval rejection while retaining all sequence/generation checks fails with delivered raced PCM. The hook advances READ_INDEX and reserves/zeroes the consumed slot while its old sequence remains published. This confirms the required non-sequence-only oracle is meaningful. Copies/logs: /private/tmp/dx434-review-mutants/ (green exit 0, sequence-only exit 1).
• All six staged generated artifacts are byte-identical to the approved artifact directory; their manifest lengths/digests match. PCM prelude equals its staged copy and has no baseline diff, as does generated source. /private/tmp/dx434-review-bytes.log.

The author's full headless result of 160 passed / 1 existing skip is retained in the issue; this independent proportional review does not claim another full headless run. No concrete bug was found within the source-spectrum observation contract. Checkout remains clean. This PASS covers the SDK observation boundary, not downstream app spectrum integration or issue closure bookkeeping.