feat(verify): output-tag preservation (@requiredTags) + render maxChars budget [FR-004]

First slice of the prompt-pillar verify extensions. Adds a cross-language,
conformance-gated static check that a template's text (body + provider-resolved
partials) contains each declared output tag as both an opening `<tag` (prefix,
attributes allowed) and a closing `</tag>`. A miss yields the new stable code
ERR_OUTPUT_TAG_MISSING (path = tag name). Reference-first: TS defines behavior
via the shared fixtures/verify-conformance corpus; C# and Python match it.

Contract decisions, pinned in fixtures:
- open tag = `<tag` followed by `>` or XML whitespace (space/tab/CR/LF) — so
  `<answer foo="1">` counts but `<answers>` does not over-match `answer`;
- close tag = exact literal `</tag>`; self-closing `<tag/>` never satisfies;
- tag text is sourced from the body + partials resolved during the single
  existing verify walk (no double resolution).

Also: render() gains an optional maxChars budget (runtime-only throw; rendered
length is data-dependent, so NOT conformance-scoped). @maxtokens stays out of
scope. New @requiredTags template attr (string[]) on template.prompt AND
template.output, wired through `meta verify` (TS + C#). Java deferred (no verify
tier yet).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dmealing

fixtures/conformance/ERROR-CODES.json

@@ -25,6 +25,7 @@
     "ERR_VAR_NOT_ON_PAYLOAD": "Build-time verify: a template variable references a field the (contextual) payload view-object does not declare.",
     "ERR_PARTIAL_UNRESOLVED": "Build-time verify: a template partial ({{> group/source}}) does not resolve in the configured provider.",
     "ERR_REQUIRED_SLOT_UNUSED": "Build-time verify (warning): a template's declared @requiredSlots slot is never referenced by the template text.",
+    "ERR_OUTPUT_TAG_MISSING": "Build-time verify: a template declares @requiredTags but its text (including provider-resolved partials) omits a required output tag's opening (<tag) or closing (</tag>) form. path = the tag name.",
     "ERR_BAD_ATTR_FILTER": "A dataGrid @filter references a non-filterable field or uses an op disallowed for the field's subtype.",
     "ERR_STORAGE_FLATTENED_ARRAY": "@storage \"flattened\" cannot be combined with isArray=true.",
     "ERR_STORAGE_WITHOUT_OBJECT_REF": "@storage was set on a field that has no @objectRef.",

fixtures/verify-conformance/verify-required-tag-in-partial/expected-drift.json

@@ -0,0 +1 @@
+[]

fixtures/verify-conformance/verify-required-tag-in-partial/options.json

@@ -0,0 +1 @@
+{ "requiredTags": ["answer"] }

fixtures/verify-conformance/verify-required-tag-in-partial/partials/answer.mustache

@@ -0,0 +1 @@
+<answer>{{content}}</answer>

fixtures/verify-conformance/verify-required-tag-in-partial/payload.json

@@ -0,0 +1,3 @@
+[
+  { "name": "content" }
+]

fixtures/verify-conformance/verify-required-tag-in-partial/template.mustache

@@ -0,0 +1,2 @@
+Please respond.
+{{> partials/answer}}

fixtures/verify-conformance/verify-required-tag-missing-close/expected-drift.json

@@ -0,0 +1,3 @@
+[
+  { "code": "ERR_OUTPUT_TAG_MISSING", "path": "answer" }
+]

fixtures/verify-conformance/verify-required-tag-missing-close/options.json

@@ -0,0 +1 @@
+{ "requiredTags": ["answer"] }

fixtures/verify-conformance/verify-required-tag-missing-close/payload.json

@@ -0,0 +1,3 @@
+[
+  { "name": "content" }
+]

fixtures/verify-conformance/verify-required-tag-missing-close/template.mustache

@@ -0,0 +1 @@
+<answer>{{content}}