Verify native subform JSON and complete field-create defaults

[Llewellynvdm]

Joomla re-encodes native subform values (escaped slashes/Unicode, whitespace and object member order), so correct writes previously became uncertain and retained their write lease. Compare the decoded outer JSON only for approval-bound native subform metadata. Preserve list order, object/list kinds, scalar types and nested literal text; refuse malformed, oversized, deeply nested or numerically lossy values.

Also complete omitted field-create description and params from Joomla's administrator form defaults in all six reviewed API/native contexts. Explicit values and PATCH behavior are preserved. This prevents the concrete missing non-null SQL payload failure reported in the paired TypeScript issue.

Ambiguous HTTP 400 responses still retain uncertainty: Joomla can persist a model and then return 400 when check-in fails. The post-save regression proves that uncertainty, lease retention and mutation-free replay remain intact. Existing uncertain executions require administrator inspection/reconciliation.

Fixes #39. Related to joomengine/joomla-mcp#57. Paired TypeScript PR: joomengine/joomla-mcp#59.

PR #40 is now merged. This PR targets main; its existing branch incorporates the merged category-assignment base with the subform and field-create fixes preserved.

Validation:

• All 35 local PHP contract suites pass, including 1,461 custom-field, 653 category-assignment and 167 field-default assertions.
• Source ZIP/relocated runtime, source-pinned seed reproducibility and changed PHP syntax checks pass.
• Added installed Joomla article subform fixtures that verify native re-encoding, independent read-back, replay without mutation and a subsequent write after lease release; installed workflow results remain the live acceptance evidence.
• Field-default installed fixtures deliberately omit description/params and verify native storage, DOM preparation, replay and update preservation.

[Llewellynvdm]

Final-head CI is green for f5030a63fa6e7246f8b606d11ae8d150b962c235:

• PHP 8.3/8.4 contracts
• Installed Joomla PHP/database matrix: all four PHP 8.3/8.4 × MySQL/PostgreSQL jobs pass
• Installed JCB golden image

The actual Joomla 6.1.3 custom-field suite passes 2,421 checks, including the new article subform's native JSON re-encoding, independently read parent value, lease release, replay without mutation and subsequent write. Source ZIP/relocated-runtime checks pass (1,880 assertions).

Paired TypeScript fix: joomla-mcp #59. Merge the category-assignment base PR #40 first, then retarget this PR to main.

[Llewellynvdm]

Resolved the conflicts with main in b618539 after #40 merged. Retained both changelog entries and implementation notes. The merge commit has exactly the same file tree as the previously green PR head (tree 1206d791714c7785200ce0e520b7014eeedf59c4), preserving all runtime and regression fixes.

Re-ran the relevant local contracts: 1,461 custom-field + 167 field-default + 653 category-assignment checks passed. Fresh Actions runs are triggered for this merge commit. Updated the PR description to remove the obsolete stacking instructions.