timholy/claude_config/limit-struct-mutability

1. Analyze mutable structs

Perform a whitespace-insensitive search for mutable struct definitions in src/ or ext/.

If any are found, report them to the user and ask if they want to run a full analysis.

If none are found, or the user declines, this task can be terminated. If it has been called from /freshen-package, the step should be marked as "DONE".

2. Analyze access patterns

Perform the analysis in a subagent. Read the code in src/, ext/, and test/. Note the full list of fields in the order they appear in the type definition, and whether each (1) is already declared const, (2) is omitted in any partial new constructor, and (3) any package code or test mutates it after construction.

The subagent should also report, for each mutable struct, any evidence that identity semantics are relied upon: uses of objectid(x), finalizer(..., x), identity-based ===/!== comparisons on instances of the type (excluding sentinel checks like === nothing), or storage in IdDict/WeakKeyDict.

The subagent should return this information as a structured list, one for each mutable struct.

3. Propose changes

In the main session, analyze the report and assess whether some fields could be marked const.

If every field of a `mutable struct` would end up `const` (combining existing and newly-proposed const fields), prefer dropping mutable and removing the const annotations entirely — unless the subagent's report shows evidence of identity-dependent use (objectid, finalizer, identity-based ===/!==, IdDict/WeakKeyDict). In that case, keep mutable and add a brief comment explaining why immutability was rejected.

When proposing this to the user, state explicitly that the change drops the heap-allocation guarantee and changes === from identity to structural equality, and summarize what the identity-usage search found.

Otherwise, for any candidate fields to be marked const, assess whether field-reordering is required: any fields to be marked as const must appear before any fields omitted in partial new constructors.

If reordering is required:

• use Julia's fieldoffset (via MCP) to determine the memory layout of the struct
• propose a reordering that aims for compact layout. Ties should be broken by preserving existing order.

Propose any changes to the user. Ask for approval before proceeding.

If the user declines, terminate the task. If it has been called from /freshen-package, the step should be marked as "DONE".

4. Implement changes

Implement any agreed-upon changes.