feat(schema): add SHOULD for capability_id on every format_options[] entry

[bokelley]

Closes #4856

Summary

product-format-declaration.json capability_id description update: sellers SHOULD publish a stable capability_id on every format_options[] entry — not just when structurally required to disambiguate. Pairs the seller-side obligation with the v2 buyer capability (PackageRequest.capability_ids[]) added in #4845, so sellers reading 3.1 release notes see the obligation alongside the capability in the same minor.

The change also:

• Fixes "should be stable once published" (informal) → Sellers SHOULD NOT change a published capability_id (normative, consistent with display_name's pattern)
• Fixes "V2" → "v2" to match file convention
• Softens "will receive UNSUPPORTED_FEATURE" → "may receive" to accurately reflect the SHOULD on the reason field in package-request.json

Non-breaking justification: Description-only change. The capability_id field remains type: string and optional in JSON Schema (required array unchanged). Existing conformant implementations remain conformant — sellers who omit capability_id on single-format_kind entries stay valid; the SHOULD creates a normative recommendation, not a mandatory requirement.

Not patch-eligible for 3.0.x. The prior description was "Recommended for any declaration that may be referenced by capability_id over time" (informal). Adding explicit RFC 2119 SHOULD language fails the patch-eligibility test (a conformant 3.0.0 implementation that omits capability_id entirely is valid but does not satisfy the new SHOULD). Ships on main as 3.1.0-beta.N under pre-mode — correct and intentional.

Pre-PR review:

• code-reviewer: approved — no blockers; two wording nits addressed (lowercase "should" → normative, "will receive" → "may receive")
• ad-tech-protocol-expert: approved — non-breaking per spec; minor changeset correct; REQUIRED + SHOULD combination is coherent and consistent with existing patterns in this schema

Triage-managed PR. This bot does not currently iterate on
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See #3121
for context.

Session: https://claude.ai/code/session_017v9EZzS9ZFzexe58cmFc4k

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Generated by Claude Code

[bokelley]

Closing as superseded by current main. The capability_id SHOULD guidance and changeset are already present in static/schemas/source/core/product-format-declaration.json and .changeset/should-publish-capability-id-on-every-format-option.md; main also has newer adjacent format_schema mirror-domain text, so resolving this branch would only reintroduce stale wording.