Resolve three validation details before adopting packet 0.5 proposals #92

Follow-up from PR #22 at 89372a0, covering three Codex findings. These proposals are unapproved and must not be emitted by the app. This issue does not change the current contract.

Before adopting the affected proposals, the contract owners should settle:

• Ground measurements must remain interpretable when the meter anchor changes. Choose an anchor-relative measurement or record enough time/frame information to associate the measurement with its anchor pose. Review finding, packet/README.md:36.
• Estimated-depth uncertainty needs validation beyond the stated nominal layout. Require the expected dimensions and sample count, finite non-negative values, and explicit handling of invalid or missing depth. Review finding, packet/README.md:63.
• A mesh that declares classification must carry its face-classification property. State when omission is allowed and reject a mismatch between the declaration and the payload. Review finding, packet/validator-checks.md:55.

Acceptance: record the owners’ decisions before adopting these fields. If implemented, add isolated tests for anchor changes, malformed uncertainty arrays, and declared-but-missing classifications.

Disposition for #22: deferred documentation details under the manager’s convergence rule. The PR adds no producer, parser, or placement-rule implementation, and explicitly prohibits sending unapproved fields. These are adoption prerequisites, not verified P1 or current executable safety defects.