chore: align package README names, harden headers getter, simplify forEach loops

[spydon]

What

A small batch of cleanups in the same spirit as #1458 / the recent functions_client simplifications.

• README headings: gotrue, realtime_client, storage_client, and supabase still used the old *-dart repo slugs in their first heading. Aligned them to the actual pub package name (matching the earlier functions-dart → functions_client fix).
• SupabaseClient.headers: was get headers { return _headers; }, returning the internal map directly. Now returns Map.unmodifiable(_headers) with arrow syntax, mirroring functions_client. This prevents callers from mutating the returned map and silently desyncing from the internal state (header changes must go through the headers= setter, which propagates to all sub-clients).
• forEach closures → plain loops:
  • postgrest_filter_builder.match(): dropped the reassign-inside-closure query.forEach((k, v) => url = ...) for a for loop.
  • realtime_client transformers.convertChangeData: record.forEach((key, value) {...}) had an unused value parameter → for (final key in record.keys).

Why

Consistency across package READMEs, and closing a small footgun on the public headers getter so external mutation can't quietly diverge from what the sub-clients actually send.

Verification

• dart format --set-exit-if-changed: clean
• dart analyze lib on postgrest, realtime_client, supabase: no issues
• Tests: supabase client_test (incl. Headers Management), utilities_test, mock_test, and realtime transformers_test pass. Pre-existing integration tests that require a local server are unaffected.

Note on the headers change

This is technically a behavior change on a public getter: external client.headers['x'] = y previously no-op'd (the mutation was lost) and now throws. The supported path to change headers remains the headers= setter.