chore(cli): R1 follow-ups — add-flag test, webm-alpha telemetry, subcomp-media comment [P2]

[miguel-heygen]

• fix(cli): add --no-clipboard throws 'Unknown flag: --clipboard' [P2] #2067: packages/cli/src/utils/reject-unknown-flags.test.ts adds coverage for add --no-clipboard flag negation.
• fix(cli): warn when a WebM render silently drops its alpha channel [P2] #2044: packages/cli/src/telemetry/events.ts and packages/cli/src/utils/webmAlphaCheck.ts emit telemetry when WebM alpha is dropped.
• fix(engine): relative data-start on <audio> silently drops the track [P1] #2062: packages/producer/src/services/htmlCompiler.ts documents the numeric data-start invariant for browser media discovery.

[miguel-heygen]

Closing in the backlog value sweep. This is documentation, test-only, dependency upkeep, refactoring, or narrow diagnostic/ergonomic polish without enough current user impact or an active merge path to justify carrying it open. Please reopen or resubmit against current main if the need is still concrete.