feat(fs): add a read render-intent card for the read tool result

The read tool's result carries structured numbered lines, but only the
model-facing envelope text reached the client. Add a card:'read' result view
(ReadResultView) projecting {path, lines, totalLines, lang} through the tool's
output.presentationMeta so presentResult reproduces it on live and replay
paths; the pending call stays a generic read card. A UI without the read
capability falls back to the envelope-stripped content, so the TUI is
unchanged. The web consumer that renders the line-numbered view is a follow-up.

packages/fs/tool-fs/tests/read-render.spec.ts

@@ -6,7 +6,7 @@
  */
 
 import { describe, expect, it } from 'vitest'
-import { buildWindow, READ_MAX_BYTES, READ_MAX_LINE_LENGTH } from '../src/read-render.ts'
+import { buildWindow, langFromPath, readMetaFromMeta, READ_MAX_BYTES, READ_MAX_LINE_LENGTH } from '../src/read-render.ts'
 import type { ReadWindow } from '../src/read-render.ts'
 
 const DEFAULT_CAPS = { maxLineLength: READ_MAX_LINE_LENGTH, maxBytes: READ_MAX_BYTES }
@@ -116,3 +116,54 @@ describe('buildWindow', () => {
     })
   })
 })
+
+describe('langFromPath', () => {
+  it('maps a known extension to its language hint, case-insensitively', () => {
+    expect(langFromPath('src/a.ts')).toBe('ts')
+    expect(langFromPath('src/a.TSX')).toBe('tsx')
+    expect(langFromPath('/abs/module.mjs')).toBe('js')
+    expect(langFromPath('conf.yml')).toBe('yaml')
+    expect(langFromPath('README.md')).toBe('md')
+  })
+
+  it('reads the extension after the last path segment and last dot', () => {
+    expect(langFromPath('a.py.bak')).toBeUndefined()
+    expect(langFromPath('archive.tar.gz')).toBeUndefined()
+    expect(langFromPath('/dir.py/plain')).toBeUndefined()
+    expect(langFromPath('C:\\src\\main.rs')).toBe('rs')
+  })
+
+  it('returns undefined for a dotfile, an extensionless name, and an unknown extension', () => {
+    expect(langFromPath('.gitignore')).toBeUndefined()
+    expect(langFromPath('/etc/hosts')).toBeUndefined()
+    expect(langFromPath('data.unknownext')).toBeUndefined()
+    expect(langFromPath('trailingdot.')).toBeUndefined()
+  })
+})
+
+describe('readMetaFromMeta', () => {
+  const good = { path: '/abs/a.ts', lines: [{ number: 1, text: 'x' }], totalLines: 1, lang: 'ts' }
+
+  it('narrows a well-formed read meta, with and without a lang hint', () => {
+    expect(readMetaFromMeta(good)).toEqual(good)
+    const noLang = { path: '/abs/a', lines: [], totalLines: 0 }
+    expect(readMetaFromMeta(noLang)).toEqual(noLang)
+  })
+
+  it('returns undefined for absent, non-object, or array meta', () => {
+    expect(readMetaFromMeta(undefined)).toBeUndefined()
+    expect(readMetaFromMeta(null)).toBeUndefined()
+    expect(readMetaFromMeta('nope')).toBeUndefined()
+    expect(readMetaFromMeta([good])).toBeUndefined()
+  })
+
+  it('returns undefined when a field is missing or the wrong type (defensive narrowing)', () => {
+    expect(readMetaFromMeta({ ...good, path: 5 })).toBeUndefined()
+    expect(readMetaFromMeta({ ...good, totalLines: '1' })).toBeUndefined()
+    expect(readMetaFromMeta({ ...good, lines: 'nope' })).toBeUndefined()
+    expect(readMetaFromMeta({ ...good, lines: [{ number: '1', text: 'x' }] })).toBeUndefined()
+    expect(readMetaFromMeta({ ...good, lines: [{ number: 1 }] })).toBeUndefined()
+    expect(readMetaFromMeta({ ...good, lines: [null] })).toBeUndefined()
+    expect(readMetaFromMeta({ ...good, lang: 5 })).toBeUndefined()
+  })
+})