refactor(fs): prune write-only fields and the dead routing knob from the seam

The fs seam split left four pieces of pre-split surface populated on
every call and read by nobody:

- STREAM_MIN_SIZE + FsIoInternals.streamMinSize in dsh-fs-local: the
  backend has no read routing (readWholeText/streamWholeText are
  separate primitives the caller picks), and the real 10 MiB routing
  constant lives in dsh-tool-fs's read tool. Delete the dead mirror and
  the knob whose JSDoc claimed an override that did not exist; the
  remaining FsIoInternals knobs stay (the atomic-write tests use them).
- FsTarget.inputPath: a "diagnostics only" field every backend and test
  fake had to fabricate, with zero production readers (policy and error
  messages use targetKey/displayPath). listDir gave children the bare
  entry name, which was nobody's input.
- FsEditOutcome.replacements/.replaceAll: replacements had no reader
  (the single-match policy is enforced by the FS_AMBIGUOUS_EDIT /
  FS_EDIT_NOT_FOUND throws, whose message keeps the internal count);
  replaceAll only echoed the replace_all argument back to
  formatEditOutput, which now takes it from the parsed args. The
  outcome shrinks to { version, before, after }, parallel to
  FsWriteOutcome's backend-discovered fields. Emitted text is unchanged
  for both branches (no snapshot churn).
- FileReadOutcome.limit/.version: formatReadOutput renders
  offset/lines/totalLines/truncatedByBytes only, and the fs/observed
  emit uses info.version directly.

Backends shed four fabrication obligations and gain none. Doc pastes
(core-data-structures/filesystem.md), the dsh-fs README resolve row,
and the test fakes shrink with the types. RFC moved to
implemented/simplification and amended to the shipped shape
(FsEditSpec -> FsEditRequest name fix; manifest rows needed no change).

packages/fs/tool-fs/tests/tools.spec.ts

@@ -40,7 +40,7 @@ class FakeFs extends FileSystem {
   }
 
   override async resolve(path: string): Promise<FsTarget> {
-    return { inputPath: path, targetKey: FsTargetKey(`key:${path}`), displayPath: `/abs/${path}` }
+    return { targetKey: FsTargetKey(`key:${path}`), displayPath: `/abs/${path}` }
   }
   override async stat(target: FsTarget): Promise<FsInfo | undefined> {
     this.throwIfArmed()
@@ -71,7 +71,7 @@ class FakeFs extends FileSystem {
     const content = this.files.get(target.targetKey) ?? ''
     const after = content.split(edit.oldString).join(edit.newString)
     this.files.set(target.targetKey, after)
-    return { replacements: 1, replaceAll: edit.replaceAll, version: FsVersion('v3'), before: content, after }
+    return { version: FsVersion('v3'), before: content, after }
   }
 }
 
@@ -252,7 +252,7 @@ describe('read tool', () => {
 })
 
 describe('formatReadOutput footer variants', () => {
-  const base: FileReadOutcome = { offset: 1, limit: 2000, lines: [{ number: 1, text: 'x' }], totalLines: 1, version: FsVersion('v') }
+  const base: FileReadOutcome = { offset: 1, lines: [{ number: 1, text: 'x' }], totalLines: 1 }
 
   it('reports a byte-capped read', () => {
     const out = formatReadOutput('/f', { ...base, totalLines: 99, truncatedByBytes: true })