Tooling: audit private exports across both pinned profiles

[chrisbbreuer]

Parent: #140

The public C audit previously subtracted implemented private exports using only the Home inventory. This misclassified Bun-only entries and shared support exports that are intentionally outside Home declarations.

Acceptance:

• Classify private exports against the union of the pinned Home and Bun inventories.
• Keep support-only private exports explicit rather than silently ignoring arbitrary names.
• Preserve the 117-function public and 19-extension denominators.
• Make zig build c-api-audit green with an exact private-export count.

[chrisbbreuer]

Complete on main at cbc26bf0. The public audit now unions both pinned private inventories and an explicit closed set of three BunString retain/release support exports. zig build c-api-audit is green at 117/117 public, 19 extensions, and 178 classified private/support exports.