fix(wake-briefing): scan the shared debug.log once at network scope, not per site

[chubes4]

Fixes #3522

The bug

WordPress multisite shares one wp-content/debug.log file across every site — WP_CONTENT_DIR is network-wide and WakeBriefingTask::resolveDebugLogPath() never reads a per-site option. But gatherNetworkSignals() ran gatherSiteSignals() — which unconditionally called getPhpFatals() — once per blog under switch_to_blog(). On the live 11-site extrachill.com network this meant the identical fatal signature was scanned, parsed, and rendered 11 times, once per site line:

- **extrachill.com** — ⚠ 9 PHP fatal(s) in debug.log, top: "owner-reference.php: Cannot redeclare function ec_link_page_owner_compatibility…" ×8.
- **community.extrachill.com** — ⚠ 1 error(s) logged, top signature ×1: "TaskScheduler: queued task requires agent context". · ⚠ 9 PHP fatal(s) in debug.log, top: "owner-reference.php: Cannot redeclare function ec_link_page_owner_compatibility…" ×8.
- **shop.extrachill.com** — ⚠ 9 PHP fatal(s) in debug.log, top: "owner-reference.php: Cannot redeclare function ec_link_page_owner_compatibility…" ×8.
- **artist.extrachill.com** — ⚠ 1 error(s) logged, top signature ×1: "TaskScheduler: queued task requires agent context". · ⚠ 9 PHP fatal(s) in debug.log, top: "owner-reference.php: Cannot redeclare function ec_link_page_owner_compatibility…" ×8.
- **events.extrachill.com** — ⚠ 2 task type(s) failing repeatedly — `unknown` ×128, `extrachill_network_classify_post_terms` ×6. Investigate? · ⚠ 15 job(s) stuck in processing. · ⚠ 342 error(s) logged, top signature ×134: "Job marked as failed". · ⚠ 9 PHP fatal(s) in debug.log, top: "owner-reference.php: Cannot redeclare function ec_link_page_owner_compatibility…" ×8.
... (6 more sites, same fatal clause repeated on every line)

~1KB of a 2,389-byte always-on WAKE.md file was one repeated string — roughly 40%. The volume alone is bad; the signal loss is worse: events.extrachill.com genuinely has 342 logged errors, 15 stuck jobs, and 2 task types failing repeatedly (128 + 6 occurrences) — and that line read with the exact same visual weight as the ten duplicate-noise lines around it. The briefing's whole job is a 3-second glance at "anything red that happened recently"; repeating one signature 11× worked directly against that.

Note: the underlying fatal (ec_link_page_owner_compatibility redeclaration) is a separate, already-shipped fix (artist-platform v1.21.1). This PR is only about the briefing reporting one fatal eleven times.

The fix: network-scope scan, not post-hoc dedup

Two approaches were on the table:

1. Post-hoc dedup — scan per-site as before, then detect signatures identical across all (or most) sites and collapse them.
2. Network-scope scan — recognize that debug.log is structurally shared, not coincidentally identical, and scan it exactly once at network scope instead of once per site.

Chose (2). A same-vs-different threshold in approach (1) has no meaningful "different" case to guard against — the file is quite literally the same file regardless of which blog is switched-to, so it would always compare identical, on every run, forever. Approach (1) would also still pay the O(sites) cost of reading and parsing an up-to-5MB tail N times, just to discover N identical strings and throw N−1 of them away. Approach (2) fixes both the noise and the wasted I/O: getPhpFatals() now runs once per network briefing, not once per site.

Implementation: gatherSiteSignals() gains a $scan_fatals flag (default true, so single-site-scope behavior — the far more common non-multisite case — is untouched). gatherNetworkSignals() calls getPhpFatals() once before the per-site loop, hoists any result into a **network-wide** line, and passes $scan_fatals = false into each per-blog gatherSiteSignals() call so the identical scan never runs again inside the loop.

Threshold: none needed, by construction — see above. This isn't "hoist when N/N sites agree," it's "this fact was never a per-site fact to begin with."

Placement: the network-wide line renders first, ahead of the per-site lines. Rationale: it's the most network-wide-relevant fact and a reader scanning top-to-bottom should see it before wading into 11 site lines, several of which are otherwise quiet and omitted entirely.

Before / after

Before (11 identical clauses, shown truncated above) — events.extrachill.com's real signal buried under 10 copies of the same noise.

After:

- **network-wide** — ⚠ 9 PHP fatal(s) in debug.log, top: "owner-reference.php: Cannot redeclare function ec_link_page_owner_compatibility…" ×8.
- **community.extrachill.com** — ⚠ 1 error(s) logged, top signature ×1: "TaskScheduler: queued task requires agent context".
- **artist.extrachill.com** — ⚠ 1 error(s) logged, top signature ×1: "TaskScheduler: queued task requires agent context".
- **events.extrachill.com** — ⚠ 2 task type(s) failing repeatedly — `unknown` ×128, `extrachill_network_classify_post_terms` ×6. Investigate? · ⚠ 15 job(s) stuck in processing. · ⚠ 342 error(s) logged, top signature ×134: "Job marked as failed".

One shared fact stated once; events.extrachill.com's real signal now stands on its own with nothing competing for attention on the same line.

Verification

• php -l on both touched files — clean.
• No PHPUnit-suite (*Test.php) coverage exists for WakeBriefingTask; it's covered by dependency-free "smoke" scripts under tests/*-smoke.php (existing pattern — see wake-briefing-infra-signals-smoke.php, wake-briefing-credential-integrity-smoke.php). Added tests/wake-briefing-network-fatals-dedupe-smoke.php, run via php tests/wake-briefing-network-fatals-dedupe-smoke.php:
  • Builds a 3-site fixture (two quiet sites + one site with genuinely site-specific facts mirroring events.extrachill.com's shape) and a shared temp debug.log.
  • Asserts gatherNetworkSignals() produces exactly one **network-wide** line, that it renders first, that no per-site line repeats the fatal text, and that the genuinely site-specific facts still render correctly on their own site line.
  • Asserts single-site scope (gatherSiteSignals($since), the default) still includes fatals directly — regression guard for the non-multisite path.
  • 8/8 assertions pass.
• Re-ran the two pre-existing wake-briefing-*-smoke.php scripts (infra signals: 13/13, credential integrity: 25/25) — both pass unchanged, confirming the $scan_fatals default preserves existing single-site behavior exactly.

Constraints followed

• Worked only in this worktree, branch fix/3522-wake-md-network-dedupe.
• No CHANGELOG.md edit, no version bump.
• No release/deploy.