[Feature] applyDefaults helper for repeated `settings.x ?? DEFAULT_X` pattern

[pathosDev]

Size / Priority

• Size: Trivial (~30+ sites)
• Category: C.2 Simplifications & DRY.
• Risk: low.

Affected files

• Across the codebase — sites using settings.x ?? DEFAULT_X to apply per-call defaults.

Background

The pattern is everywhere:

const timeoutMs = options.timeoutMs ?? DEFAULT_ASK_TIMEOUT_MS;
const batchSize = options.batchSize ?? DEFAULT_BATCH_SIZE;
const consistency = options.consistency ?? 'local';

For methods with many optional parameters, this becomes verbose:

async updateAsync(opts) {
  const timeoutMs = opts.timeoutMs ?? 5_000;
  const consistency = opts.consistency ?? 'local';
  const pendingId = opts.pendingId ?? nextPendingId();
  // ... etc
}

Could be:

async updateAsync(opts) {
  const resolved = applyDefaults(opts, UPDATE_DEFAULTS);
  // resolved.timeoutMs, resolved.consistency are guaranteed populated
}

(applyDefaults from #255.)

Target

Same helper as #255 (applyDefaults<S>(partial, defaults)):

• Returns a fully-populated S from a Partial<S> + a S defaults.
• TypeScript narrows S correctly.

Cleaner than per-property nullish-coalescing.

Integration / risk

• No behavioural change.
• Cleaner call sites.
• Default sets centralised (paired with [Feature] Centralize magic-number constants #257 — magic-number constants).

Test plan

1. Per-site regression: behaviour unchanged.
2. Edge cases: explicit undefined vs missing field handled consistently.

Acceptance criteria

• applyDefaults helper (shared with [Feature] resolveSettings helper for DRY settings-merge boilerplate #255) used across all sites.
• Per-method Defaults constants extracted.
• Tests pass.
• No CHANGELOG entry needed.
• Coordinate with [Feature] resolveSettings helper for DRY settings-merge boilerplate #255 + [Feature] Centralize magic-number constants #257.

[pathosDev]

Closing as duplicate of #255 (resolveSettings/applyDefaults helper). The 'default values repeated at call sites' pattern is exactly what applyDefaults solves — fixed there.