Show the cache hit rate after the cold start, and the output ratio

The session's first executor request pays for the prefix every later one
reuses, so the session-wide hit rate mixes "did the cache work once warm"
with "how much a cold start could reuse". usagereport already records the
cold start, but nothing a live session reads projected it.

The live figures come from the serve ledger, which restarts on /new,
/resume and /fork; usageTally does not, so its ColdStart can belong to an
earlier conversation in the same process. The broadcaster therefore takes
the cold start from the ledger's own population: the first request the
ledger files under the executor (pricing.LedgerSource, the rule Add
already used), split by the same CacheSplit the ledger applies, and reset
with it. SessionUsage carries it as coldStart.

- /status (TUI): "warm cache" row, ledger hit rate with the cold start
  taken out of hit, miss and unknown; "out/in" row, output ÷ input.
- Desktop session overview behind the cache meter: the same two figures,
  computed in state/overview beside cacheRate.
- wire-parity: ColdStartUsage mirrored in port/session.ts.

tools/repolint/wireparity.go

@@ -122,6 +122,7 @@ var mirroredWireTypesHead = []wireMirror{
 	{"internal/frontend/serve/subagents.go", "subagentRunView", tsSubagentFile, "SubagentRun"},
 	{"internal/frontend/serve/subagents.go", "subagentTranscriptView", tsSubagentFile, "SubagentTranscript"},
 	{"internal/frontend/serve/session_usage.go", "SourceUsage", tsSessionFile, "SourceUsage"},
+	{"internal/frontend/serve/session_usage.go", "ColdStartUsage", tsSessionFile, "ColdStartUsage"},
 	// The agent's tabs. The target id is what the window draws a view by, so a
 	// tab the page cannot read one for is a tab it cannot show.
 	{"internal/platform/browser/session.go", "TabInfo", tsSessionFile, "BrowserTab"},