Rationalize proposals/: consolidate design docs into RFCs

[akollegger]

Overview

Cleans up the proposals/ tree so it has a single clear organizing principle. After the earlier RFC restructure, the same designs lived in two places (older top-level proposals and the numbered RFCs that superseded them), the RFC index had a broken link, and supporting notes were mixed in with authoritative RFCs. This PR consolidates everything, with no design content lost — anything unique was migrated into RFC appendices before its source file was removed.

Final structure

proposals/
├── graph-value-instances.md   # pre-RFC stub
├── pattern-equivalence.md      # pre-RFC stub
├── research/                   # NEW: non-RFC supporting notes
│   └── (graph-lens, gram-hs-cli-*, pattern-basic-aspects-review, pattern-matching-dsl-design)
└── rfc/                        # README + RFC-001..010 only

What changed

Removed (fully superseded, nothing unique lost):

• 8 top-level proposals already absorbed by RFCs (graph-classifier, graph-query, graph-mutation, graph-transform, pipeline-scenarios, scope-unification-proposal, representation-map-proposal, pattern-reconciliation)
• SEMANTICS.md and EXTENDED-SEMANTICS.md (fully covered by RFC-003)

Migrated then removed (had unique content):

• pattern-graph.md → RFC-004 Appendix A (PatternGraph container design)
• DESIGN.md → RFC-002 Appendix B (deferred design sketches behind the open questions)
• pattern-category.md → RFC-002 Appendix A (value magma, layered CategoryLens, worked example, applications)

Reorganized:

• Moved 5 non-RFC docs into a new proposals/research/ directory
• Elevated graph-value-instances.md to an indexed pre-RFC stub

Fixed references:

• Repaired the broken RFC-001 README row (was pointing at RFC-001-strata-and-aspects.md with pre-rename "strata/aspects" terminology) — the last lingering trace of the strata→frames / aspects→spans rename
• Rewrote the README index with an absorbed-document provenance table and a newcomer orientation section
• De-linked all RFC Supersedes: headers that pointed at removed files (kept as plain provenance text)
• Repointed 8 specs/ Input: provenance lines to their superseding RFCs (original prompt text preserved, accurate pointer appended)

Reviewer notes

• This is docs-only — no source or test changes.
• The two commits separate the mechanical proposals/design/ → proposals/rfc/ rename from the substantive consolidation, so the diff is easier to follow commit-by-commit.
• Deletions of DESIGN.md and pattern-category.md were gated on a verification pass confirming their unique content was carried into RFC-002's new appendices; the specs/ Input: lines were intentionally left as historical records with the pointer appended rather than rewritten.
• Verified: no broken internal links remain anywhere in the repo (README links, RFC cross-references, and spec→RFC links all resolve).

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