fix(explore): shrink a later cluster into the remainder instead of dropping it (CG-36)

A file's ranked clusters were all-or-nothing past the first one: the top-ranked
cluster was taken (shrunk to fit when it had to be) and every cluster below it
was rendered whole, then either fit the remainder or was dropped entirely. On a
file whose top-ranked cluster is TRIVIAL that discards the answer — django's
`db/models/sql/query.py` kept a 22-line glue cluster and dropped the 624-line
`Query` body, spending 1,923 of a 7,947 reservation; okhttp's
`RealInterceptorChain.kt` did the same behind its import header.

The response stayed full, which is why this was invisible: the unspent
reservation carried forward exactly as designed and a file scoring a fifth as
much took the bytes.

Two sites, the same rule — hold the remainder while it is still worth a section
(CG-26's between-FILES lesson, applied between CLUSTERS):

- selection now shrinks a later cluster into what is left of the file's budget,
  by the same whole-member rule the first cluster already used;
- the ceiling trim re-renders the weakest cluster into the room that remains
  before dropping it. On excalidraw's `typeChecks.ts` the section-cost estimate
  missed by 13 chars and a 1,512-char cluster — the file's highest-SCORING one —
  was thrown away to pay for it.

Cluster RANKING is untouched: measured, both real cases lost on `maxImportance`,
not on the density tiebreak the issue suspected, and density-first is what keeps
Alamofire's `Session.swift` from burying its methods under the property list.

Suite (6 repos, clean-rebuilt indexes): all 8 starvation flags cleared,
+1,012 source chars net. django's `sql/query.py` 1,923 -> 10,082 of 7,947,
okhttp's `RealInterceptorChain.kt` 1,474 -> 6,038 of 6,058, gin's
`routergroup.go` 3,273 -> 5,632. okhttp trades its rank-6 file (score 21) for
+7,196 chars in the two files that answer the question.

Ships two fixtures pulling in opposite directions (`starved-cluster-ts` and
`dense-header-ts`), a `spendShareAtLeast` gate in probe-allocation, and
probe-file-spend.mjs — a standing per-file reservation-vs-delivered sweep.

__tests__/fixtures/starved-cluster-ts/src/app/client.ts

@@ -0,0 +1,24 @@
+import { RequestChain, describeChain } from '../pipeline/chain';
+import type { PipelineRequest, PipelineResponse } from '../pipeline/types';
+import { openSocket } from '../transport/socket';
+
+/**
+ * The entry point a caller reaches for. Everything the chain does happens
+ * underneath this call, which is why a flow question names it.
+ */
+export async function sendRequest(request: PipelineRequest): Promise<PipelineResponse> {
+  const socket = openSocket(request.host, request.port);
+  const chain = new RequestChain(request, socket);
+  trace(describeChain(chain));
+  return chain.proceed(request);
+}
+
+export function trace(line: string): void {
+  if (process.env.PIPELINE_TRACE) process.stderr.write(`${line}\n`);
+}
+
+export async function sendAll(requests: PipelineRequest[]): Promise<PipelineResponse[]> {
+  const out: PipelineResponse[] = [];
+  for (const request of requests) out.push(await sendRequest(request));
+  return out;
+}

__tests__/fixtures/starved-cluster-ts/src/app/config.ts

@@ -0,0 +1,14 @@
+export interface ClientConfig {
+  host: string;
+  port: number;
+  retries: number;
+  userAgent: string;
+}
+
+export function defaultConfig(): ClientConfig {
+  return { host: 'localhost', port: 8080, retries: 3, userAgent: 'pipeline/1.0' };
+}
+
+export function withHost(config: ClientConfig, host: string): ClientConfig {
+  return { ...config, host };
+}