fix(soak): apply initial favorites corpus

[tcconnally]

Problem

The manual Favorites filter was applied after playback had already started. In the 1-cell pilot, Filter: FAVORITES (67 items) appeared about one minute into a two-minute phase, leaving the first half on the full 801-item library. That is not fixed-corpus evidence.

Change

• Add a pure, tested initial soak-filter helper.
• Add opt-in HYPERWALL_SOAK_FILTER=favorites behavior that selects Favorites immediately after metadata loads and before cell-start timers run.
• Keep self.all_items intact so normal A/F controls still work.
• Record the effective soak filter in run.env.
• Clear the filter in normal launches so everyday playback is unchanged.

This supports fixed-corpus evidence for #75, #76, and #79. It does not close those issues and does not change Windows/Linux defaults.

Verification

• python3 tests/run_all.py: PASS
• python3 -m compileall -q hyperwall tests: PASS
• bash -n soak_wall.sh launch.sh: PASS
• Complete committed diff SHA-256: ed8f6525914fbdb38fd3c3fc22c195930c1e1dc2778451066634d31b518bb4e2
• Reviewed path bytes match the committed candidate.
• Native runtime qualification remains separate.