Harden event-watcher boot: resolve boot ledger in the retried poll loop

[AquiGorka]

Problem

The event-watcher could be permanently disabled by a single transient RPC failure at boot. start() resolved the boot start ledger via an un-retried getHealth() RPC (resolveBootStartLedger, taken when BOOT_SYNC_START_LEDGER_BLOCK is unset / "sync-all") before scheduling the poll loop. If that RPC blipped, start() rejected before scheduleNext(), the poll loop never ran, the error was logged-and-swallowed by startEventWatcher, and the watcher stayed dead for the life of the process — blinding the provider to all live on-chain events (provider add/remove, channel enable/disable) until a manual restart. Poll-loop errors, by contrast, were already caught and retried; only the start path was terminal.

Fix

Move boot-ledger resolution out of start() and into the first poll(), inside the existing try/catch + retry machinery. A transient failure resolving the boot position is now caught and retried on the next tick — exactly like any poll error — instead of rejecting start(). start() no longer makes an un-retried network call; the in-memory cursor stays null until the first poll resolves it.

Semantics preserved

• Boot-sync from oldest available, or the pinned BOOT_SYNC_START_LEDGER_BLOCK override — never falls back to "latest".
• Converge-by-query stays the recovery path; no durable-cursor behaviour changed.
• This is purely about making the start path retry instead of dying.

Test

EventWatcher - transient boot-ledger failure retries instead of killing the watcher: the first getHealth() throws → the watcher does not die → on RPC recovery it resolves the boot ledger, polls, and processes a provider_added event; the cursor advances. Verified failing against the pre-fix code and passing after.

Verification (Deno 2.8.2, the CI toolchain)

• deno fmt --check — clean
• deno lint — clean
• event-watcher suite — 56 passed
• full unit suite — 133 passed
• CI pay suite (src/http/v1/pay/) — 116 passed

Patch version bump 0.9.6 → 0.9.7 in this PR.