test(fitbit): require explicit fixture runtime

[kachofugetsu09]

Summary

• require AKASHIC_PLUGIN_FIXTURE_PYTHON before staging the Fitbit artifact
• mount the service .venv from the explicitly selected artifact Python instead of sys.executable
• keep missing configuration fail-loud and prove it performs zero staging writes
• export the absolute fixture Python in CI after the venv is created

Verification

• Python suite: 59 passed
• panel suite: 12 passed
• pyright: 0 errors, 0 warnings
• Plugin API v3 contract at 4dd69dd621e029e51e99aa428443fa3a4ec1f6cf: passed
• compileall and git diff --check: passed
• Terra xhigh exact review: APPROVE, no P0/P1/P2

Stack

• base: codex/content-context-20260823 / PR feat(fitbit): compose health Content and Wake context #6 exact a7ba644d87712727c0e5e6c8047f0783a1d65dc1
• head: b81978474a630a13cb8d9bd6f2fa17afb28b2d11

Recovery

• bundle: /mnt/data/coding/backups/fitbit-h5-fixture-20260824/fitbit-pr6-a7ba644d.bundle
• SHA-256: ca1cf7f9eda523bc1a8a6d73c4cf137b8a54608f02297e09712922ffee1d016e

No Core, cache, formal workspace, or deployment state was modified.