feat(treesitter): add ,imports and ,signatures flags to :symbol fences

[phil-scott-78]

What

Two opt-in, comma-combinable flags for :symbol code fences:

• ,imports — prepends the file's top-of-file using/import/require lines above the snippet (all of them, blank-line separated). Composes with ,bodyonly; no-op for whole-file embeds and import-less languages (Ruby).
• ,signatures — elides member bodies to { … } (… for non-brace bodies) for an at-a-glance outline of a type's shape. Inverse of ,bodyonly.

```csharp:symbol,imports
examples/FocusedCodeSamplesExample/MonolithWordCounter.cs > MonolithWordCounter.CountWords

examples/FocusedCodeSamplesExample/ModularWordCounter.cs > ModularWordCounter

## Why

A member-scoped fence dropped the file's import context, so readers couldn't tell where types resolved from — the docs even shipped a standing "no `using` prepend" caveat, now retired. And embedding a whole type dumped every body, with no way to show just its surface.

## How

Both extend the existing **config-driven tree walk** (no `.scm` queries):

- Lifted the shared "descend transparent wrappers, match a node-type set" descent out of `NamePathResolver` into a reusable `TreeWalker`, now driving name resolution, import collection (`ImportCollector`), and outline elision.
- Flags flow through parsing → service → extractor in a new `FragmentOptions` record, replacing the bare `bodyOnly` bool.
- Per-language `ImportNodeTypes` seed import collection (`using_directive`, `import_statement`, `use_declaration`, …).
- The outline transform splices bodies out using start-position + body-length math (no byte-offset dependency).
- Scope: `,imports`/`,signatures` apply to `:symbol` only — `:symbol-diff` still honors just `,bodyonly`.

## Docs

New how-to sections (with live-rendered examples), updated fence grammar + suffix-forms table in `code-block-args.md`, and retired the false caveat in the docsite authoring guide.

## Testing

- `dotnet test Pennington.slnx` — **51** TreeSitter (11 new) + **806** unit + **48** integration passing.
- `dotnet run --project docs/Pennington.Docs -- build` — 458 pages, no unresolved references; confirmed on the rendered page that `,imports` prepends the using and `,signatures` elides bodies, with zero extraction errors.