fix(host): release the pricing file source with the fiber, not at apply time

[rezon-aki]

Problem

The pricing.json override never applies. /dsh-usage-chart/pricing reports source: "builtin" for every model even when the file exists and parses, and editing the file has no effect (the watcher never fires either).

Root cause

src/index.ts:

ctx.effect(() => fileSource.dispose(), 'dsh-usage-chart: pricing file watcher')

That is an expression-bodied arrow: it calls dispose() while the fiber is being built and registers undefined as the disposer. The file source is destroyed before its first read, so its entries stay {} forever and ensureWatcher() is skipped.

Fix

One line — return a disposer instead of calling it:

ctx.effect(() => () => fileSource.dispose(), 'dsh-usage-chart: pricing file watcher')

Tests

• The mock effect in tests/core.test.mjs, tests/pricing.test.mjs and tests/rounds.test.mjs now follows cordis semantics (run setup, keep the returned disposer, release it in after()); the previous effect(setup) { setup() } mock discarded the disposer and hid this bug.
• New regression test: /pricing route serves the user pricing.json override (file source survives startup). Against the old form it fails (source === 'builtin'); with the fix it passes (source === 'file', file price wins).

Verification

npm ci → npm run typecheck → npm test: 41/41 pass (node 22, DSH 0.1.5-rc.2). The live host served deepseek-flash -> file, deepseek-v4-pro -> file from /dsh-usage-chart/pricing after the fix.