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).

docs/core-data-structures/filesystem.md

@@ -12,7 +12,6 @@ Every operation resolves a user-supplied path to an opaque backend target first.
 
 ```ts type-equiv
 interface FsTarget {
-  inputPath: string
   targetKey: FsTargetKey
   displayPath: string
 }
@@ -81,8 +80,6 @@ interface FsEditRequest {
 
 ```ts type-equiv
 interface FsEditOutcome {
-  replacements: number
-  replaceAll: boolean
   version: FsVersion
   before: string
   after: string
@@ -109,16 +106,14 @@ interface FsPolicyExec {
 
 ## Read outcome (consumer / read rendering)
 
-A text read is bounded by line window, byte cap, and backend limits. The outcome the model-facing `read` tool renders carries the file's version at read time; there is no `full`/`partial` view — authorization is freshness-based, so any windowed read can authorize a later write/edit when the file is unchanged. Read windowing and this outcome shape live in `dsh-tool-fs` (the executor that owns the read), not in the policy plugin.
+A text read is bounded by line window, byte cap, and backend limits. The outcome the model-facing `read` tool renders is purely presentational; there is no `full`/`partial` view — authorization is freshness-based (the tool emits `fs/observed` with the stat's version directly), so any windowed read can authorize a later write/edit when the file is unchanged. Read windowing and this outcome shape live in `dsh-tool-fs` (the executor that owns the read), not in the policy plugin.
 
 ```ts type-equiv
 interface FileReadOutcome {
   offset: number
-  limit: number
   lines: FileTextLine[]
   totalLines: number
   truncatedByBytes?: true
-  version: FsVersion
 }
 ```