fix: Close FDv1 fallback and FDv2 recovery correctness gaps

[joker23]

NOTE: stacked on top of #1982

Summary

Four fixes found reviewing the recovery-timer PR in this stack, before they shipped:

• Adds the missing "10s uninitialized" leg to the FDv2 fallback condition group: before the data system has ever received data, a source that hangs without producing interrupted or a changeSet now still triggers fallback after initFallbackTimeoutMs (default 10s), complementing the existing interrupted-based leg.
• Fixes a starvation bug where a repeated fallback directive from a source that never transitions off FDv2 (no FDv1 synchronizer configured) kept re-arming the recovery deadline, so it could never elapse. Now only the first directive per engagement re-arms it.
• Fixes a 'none'-payload changeSet incorrectly marking the data system initialized, and adds hasExistingData so a post-connection-mode-switch 'none' response on an already-initialized system doesn't churn through synchronizers looking for data that was never going to come.
• Fixes a resource leak where SourceManager left the active source open when every synchronizer slot was blocked.

---

Note

Overview
Tightens FDv2 data-source orchestration around fallback conditions, FDv1 directives, and what counts as “initialized.”

Uninitialized fallback (10s leg): Before the data system has ever received real data, synchronizer fallback now includes an init-fallback timer (createInitFallbackCondition, default 10s) that fires even when the source never reports interrupted—so a hung primary can still move to the next synchronizer. Real changeSets cancel it; none payloads do not. getConditions takes an initialized flag and initFallbackTimeoutMs; FDv2DataSource wires hasExistingData so connection-mode rebuilds skip this leg.

Initialization semantics: Synchronizer-phase handling no longer calls markInitialized() on type: 'none' changeSets unless hasExistingData is set (manager passes prior initialized on rebuild). That stops empty poll/304 responses from falsely completing start() or disarming the init-fallback leg.

FDv1 fallback without an FDv1 slot: handleFdv1Fallback now always halts the current run via sourceManager.fdv1Fallback() when synchronizers exist (sets INTERRUPTED if no FDv1 slot), instead of ignoring the directive. Recovery TTL is scheduled only once per engagement—repeated directives from a still-running FDv2 source no longer reset the deadline and starve return to FDv2.

Resource cleanup: SourceManager closes the active synchronizer when every slot is blocked, so transports are not left running during recovery.

Tests cover the new conditions, orchestration paths, and a dedicated FDv1FallbackRecovery.test.ts.

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[joker23]

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[joker23]

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