Resolved Frame content is unbounded even though stored content is capped

Problem

#51 implemented the 512KB cap the design doc promised, but it applies to a single version's stored document (frames.MaxContentBytes, checked in the shared publish path). The resolved form is still unbounded: Resolve merges every ancestor's slots, so a Frame that inherits from a chain of near-limit parents can compose to far more than 512KB.

The design doc originally claimed "A Frame with 100MB of inherited content is rejected at publish time, not at MCP-read time." That is still not true - the note in docs/design/2026-05-21-mcp-endpoint-design.md §6 was updated in #51 to say so honestly rather than leave the claim standing.

Why it matters

The resolved form is what actually gets served and held in memory:

• ResolveDoc walks the whole ancestor graph on every GetFrame, ResolveFrame, get_frame, and MCP resource read.
• Nothing caches it, so the cost is per request.
• With replicaCount: 1 and SQLite, a handful of concurrent reads of a deeply-inherited Frame is a memory and latency problem on a single pod.

Inheritance depth is not otherwise limited either - only cycles are rejected (CycleError), not depth or fan-out.

Suggested direction

Options, roughly in order of cost:

1. Cap resolved size at publish time: resolve the document as part of publish and reject when the composed form exceeds a limit. Catches it at write time, but makes a publish's validity depend on its ancestors, so a parent growing can retroactively invalidate a child's next publish.
2. Cap at read time: return a clear error rather than serving an enormous payload. Simple, but the failure surfaces to a reader who cannot fix it.
3. Limit inheritance depth and total ancestor count, which bounds the blast radius without a byte-level rule.

A decision is needed on which limit is the real contract before implementing; that is the substance of this issue.

Context

Raised by review of #51. The per-version cap landed there; this is the remaining half.

Start with docs/design/2026-05-21-mcp-endpoint-design.md §6 and trace ResolveDoc through publish, GetFrame, ResolveFrame, get_frame, and MCP resource reads. First settle whether the contract is a resolved-byte, depth, or ancestor-count limit; done means the chosen behavior is documented and its affected publish/read paths are covered.