fix: read catalog stock bands (availability) instead of exact counts

[virtualsms-io]

Summary

The catalog API now returns a stock band (in_stock / low_stock / out_of_stock) alongside the legacy exact per-country count, and will stop sending the count entirely soon. This PR moves every in-stock check in the server onto the band:

• Added src/stock.ts: catalogStockBand(row) reads availability when present, falls back to count thresholds for older payloads, and returns unknown (never in stock) when neither field is present. isCatalogInStock(row) wraps that into a boolean.
• CatalogCountry.count is now optional; availability is the new authoritative field.
• find_cheapest and the price-check stock guard both use the shared helper. Tool output now carries an availability band string per country instead of an exact stock integer or a bare stock: true flag.
• Docs (src/resources.ts, README.md) updated to tell agents to read availability, not count.
• Version bumped to 1.3.3 across package.json, server.json (root + packages[0]), .plugin/plugin.json, and the lockfile. This also fixes a pre-existing drift between package.json (1.3.1) and server.json (1.3.2) that the repo's own version-consistency.test.ts was already failing on.

Out of scope: rental tools (/api/v1/rentals/*, RentalCatalogService.physical_count) — those are a separate stock model and untouched.

Deviation from the original brief, flagged for review: the brief's "Tests" section assumed no test runner existed and asked for a new test/stock.test.mjs on Node's built-in runner, replacing the test npm script. That assumption doesn't hold — this repo already has a working Vitest suite (src/__tests__/**, 107 tests) wired to npm test. Replacing the test script would have silently stopped that whole suite from running in CI. Instead, the new coverage was added as src/stock.test.ts in the existing Vitest convention, and the test script/config were left untouched.

Test plan

• npm ci
• npm run build
• npm test (Vitest) — 11 files, 113 tests passed, including the newly-added src/stock.test.ts (6 tests) and the previously-failing version-consistency.test.ts (now fixed by the aligned version bump)
• grep -rnE "\.count\b|count >|count <=|'count'|\"count\"" src | grep -v -i "rental\|endpoint\|params.count\|args.count" — every remaining hit is transaction pagination (TransactionsPage.count), proxy-location result count, or the intentional legacy fallback inside src/stock.ts/client.ts's catalog mapping — none leak an exact catalog stock integer into tool output

 Test Files  11 passed (11)
      Tests  113 passed (113)

src/client.ts:685:      count: c.count === undefined || c.count === null ? undefined : Number(c.count),
src/client.ts:730:      count: Number(raw.count ?? items.length),
src/client.ts:978:      count: Number(it.count ?? 0),
src/stock.ts:23:  if (typeof row.count === 'number' && Number.isFinite(row.count)) {
src/stock.ts:24:    if (row.count <= 0) return 'out_of_stock';
src/stock.ts:25:    if (row.count <= 5) return 'low_stock';
src/tools.ts:2094:      count: page.count,
src/tools.ts:2106:          : page.count === page.limit

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