[alloutflo]

Fixes #633.

With x = "stuff", {{ x[0] }} currently renders nothing and reports an unknown property, while {{ x[0:1] }} renders s. Add an internal string resolver before the bean resolver in both default resolver chains so indexed access returns the expected character as a String.

Support positive and negative integer indices, including numeric index strings. Empty strings, out-of-range indices, and integer overflow resolve to null. Preserve non-index property/method fallback, custom resolvers, and string immutability. Indexing follows the existing UTF-16 boundaries used by string slices and the length filter; other object types and SafeString handling are unchanged. No public API, configuration, or dependency changes.

Validation

• Reproduced the original failure with a regression test on upstream 0eb936c before applying the fix.
• Focused resolver, slicing, bean/map, and eager/deferred tests: 83 passed.
• Full Maven verify with Temurin 17, 21, and 25: 2,254 tests per run, 0 failures/errors, 23 skipped. Maven runs on JDK 25 with the selected compiler/test toolchain; the 21/25 builds used fresh checkouts.
• Spotless, Checkstyle, SpotBugs, and PMD passed; independent diff review and git diff --check found no issues.
• Used JVM locale en_US. The existing FormatNumberFilterTest fails on untouched upstream under this Mac's de_AT locale and passes with en_US; no unrelated source changes were made.

Implemented with assistance from OpenAI Codex.