feat: display signed extensions in tx output and inspect command

Context

Implements #42 — show which signed extensions are available on a chain and how each is handled when submitting transactions.

Currently the CLI handles signed extensions invisibly. This change surfaces that information in:

• tx --dry-run output (after fee estimate)
• tx submission output (after status, before events)
• dot inspect extensions new sub-command

Plan

1. Move PAPI_BUILTIN_EXTENSIONS to src/core/metadata.ts

Shared between tx.ts and inspect.ts.

2. Add categorizeSignedExtensions() in tx.ts

Classifies each extension as: builtin | user (from --ext) | auto (auto-defaulted) | unknown.
Reuses existing autoDefaultForType() logic.

3. Add printExtensionsSummary() in tx.ts

Displays formatted extension list with color-coded status tags:

  Signed Extensions:
    CheckNonce                builtin
    CheckMortality            builtin
    CustomExtension           auto     (None)
    UserOverride              user     (from --ext)

4. Integrate into both tx output paths

• dry-run: after estimated fees
• submission: after status/error, before events

5. Add inspect extensions virtual target

dot inspect extensions lists all extensions with type details (value type, additionalSigned type, builtin tag).
Also adds a hint line in dot inspect pallet listing.

6. Tests

• Unit tests for categorizeSignedExtensions() in tx.test.ts

Key files

• src/core/metadata.ts — getSignedExtensions(), move PAPI_BUILTIN_EXTENSIONS here
• src/commands/tx.ts — categorization, display, output integration
• src/commands/inspect.ts — extensions virtual target
• src/core/output.ts — color constants (reuse)

Related: #42

Start with src/core/metadata.ts and its getSignedExtensions() implementation, then read src/commands/tx.ts and src/commands/inspect.ts to trace the existing output paths and autoDefaultForType() logic. Review src/core/output.ts for reusable color constants and tx.test.ts for the unit-test conventions. Done means both tx output paths show categorized extensions and dot inspect extensions lists the requested type details.