feat: prove receiver-held weights for source-visible PS6141 methods

[jxsl13]

PS6141 currently proves source-visible function consumers but rejects methods holding packed weights in their receiver. Extend sourceSummary with weightArgument: -1, packedArgument: 0, and rowsArgument: -1 for one explicit packed argument and receiver-held weights.

Require a typed struct or pointer receiver with exactly one packed slice field and numeric shape fields. Bind the actual receiver through source helper methods, retain independent indexed weight origin, and preserve fresh producer origin, one effective consumer, normal source completion, and closed argument/receiver effects. Receiver metadata cannot supply weight-origin evidence. Aliases, mutation, retention, foreign receiver calls, repeated consumption, opaque bodies, method values/expressions, promotion, implicit addressing, and extra memory/cache fields remain unsupported.

Existing tests are preserved. Add 25 parallel source method cases, config role boundaries, and advisory delivery assertions. Pointer/value positives use a readable weight-bearing source method scaffold modeled on GoAI QuantLinear's layout; they are synthetic activation-owner evidence, not a historical activation-positive workload or numerical/performance proof. Authentic GoAI Forward's cache/native/error behavior and receiver-held packed-byte protocol geometry remain outside this bounded extension toward issue 835.

Validation: scoped Go 1.26 ordinary/race PS6141, config, and runner tests passed; independent review found no blockers. Unchanged pre-commit and pre-push hooks passed, including all 2050 ordinary and race names under the default toolchain, vet/build/staticcheck, generated documentation, plugin build, benchmark smoke execution, and self-scan. No automatic fix, numerical-equivalence claim, or measured speedup is introduced.