feat(render-conformance): expand corpus to 14 fixtures — Mustache divergent surfaces

Original 4 fixtures (csv-injection, email, prompt, spreadsheet) only exercised
escaping + a tiny partial. Add 10 fixtures targeting known cross-implementation
divergent surfaces of the 4 Mustache engines we use (mustache.js / Stubble /
mustache.java / hand-rolled Python interpreter) — the gate that defends
prompt-cache stability.

New fixtures:
- render-inverted-section          ^x over empty array, empty string, false, null, missing
- render-standalone-tag-stripping  block tags (#,/,!,^,>) on their own line strip trailing newline
- render-dotted-path-lookup        {{user.profile.name}} walks nested objects
- render-parent-context-fallthrough  name resolves from enclosing context if not in inner section
- render-raw-html-bypass           {{{x}}} and {{&x}} bypass format escaper
- render-nested-partials           partial referencing partial 2 levels deep
- render-unicode-multibyte         emoji + CJK + combining marks preserved byte-for-byte
- render-empty-array-falsiness     empty array renders nothing; non-empty iterates
- render-falsy-values              false/null/empty-string/empty-array are falsy in sections
- render-trailing-newline-preservation  template w/o trailing newline → output w/o trailing newline

Cross-port status (all 4 ports' render-conformance gates green at 14/14):
- TS (mustache.js): all 10 new fixtures match expected.txt
- C# (Stubble):     all 10 new fixtures match expected.txt
- Python (hand):    all 10 new fixtures match expected.txt
- Java (mustache.java):  9/10 match; 1 documented drift in KNOWN_DRIFT.md —
  render-standalone-tag-stripping: mustache.java does NOT apply the standalone-line
  strip rule to {{! comment }} tags (TS / C# / Python all do). One extra blank line
  after the comment block. Other standalone strips (section, partial) work.
  Within-Java snapshot gate stays green; cross-port report flags it; KNOWN_DRIFT.md
  updated.

Verification:
- TS    `bun test packages/render/test/render-conformance.test.ts`  14/14 pass
- C#    `dotnet test ... --filter FullyQualifiedName~RenderConformance`  14/14 pass
- Java  `mvn -pl render test -Dtest=RenderSnapshotTest`  14/14 pass
- Py    `uv run pytest tests/render/test_render_conformance.py -q`  14/14 pass
- No regressions in any port's broader render test suite
  (TS 103/103, C# 98/98, Java 99/99, Py 81/81).

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

Author: dmealing

fixtures/render-conformance/render-dotted-path-lookup/expected.txt

@@ -0,0 +1,5 @@
+name: Ada
+email: ada@example.test
+role: admin
+missing-leaf: []
+missing-mid: []

fixtures/render-conformance/render-dotted-path-lookup/meta.json

@@ -0,0 +1 @@
+{ "format": "text", "note": "dotted-path interpolation {{user.profile.name}} walks nested objects" }

fixtures/render-conformance/render-dotted-path-lookup/payload.json

@@ -0,0 +1,12 @@
+{
+  "user": {
+    "profile": {
+      "name": "Ada",
+      "email": "ada@example.test"
+    },
+    "roles": {
+      "primary": "admin"
+    }
+  },
+  "missing": {}
+}

fixtures/render-conformance/render-dotted-path-lookup/template.mustache

@@ -0,0 +1,5 @@
+name: {{user.profile.name}}
+email: {{user.profile.email}}
+role: {{user.roles.primary}}
+missing-leaf: [{{user.profile.missing}}]
+missing-mid: [{{user.unknown.name}}]

fixtures/render-conformance/render-empty-array-falsiness/expected.txt

@@ -0,0 +1,5 @@
+filled:
+- one
+- two
+empty:
+end

fixtures/render-conformance/render-empty-array-falsiness/meta.json

@@ -0,0 +1 @@
+{ "format": "text", "note": "empty array section renders nothing; non-empty array renders body per item" }

fixtures/render-conformance/render-empty-array-falsiness/payload.json

@@ -0,0 +1,4 @@
+{
+  "filled": ["one", "two"],
+  "empty": []
+}

fixtures/render-conformance/render-empty-array-falsiness/template.mustache

@@ -0,0 +1,9 @@
+filled:
+{{#filled}}
+- {{.}}
+{{/filled}}
+empty:
+{{#empty}}
+- {{.}}
+{{/empty}}
+end

fixtures/render-conformance/render-falsy-values/expected.txt

@@ -0,0 +1,8 @@
+true-flag: [body]
+false-flag: []
+null-value: []
+empty-string: []
+non-empty-string: [body]
+empty-array: []
+non-empty-array: [1;2;]
+object-with-name: [Ada]

fixtures/render-conformance/render-falsy-values/meta.json

@@ -0,0 +1 @@
+{ "format": "text", "note": "section truthiness — false, null, empty string, empty array are falsy; non-empty arrays iterate; non-empty objects render once" }