Pre-size valdi_core bridge dictionaries and maps

[ultramiraculous]

[iOS] Pre-size valdi_core bridge dictionaries and maps

Description

We ran into a workload that pushes a large number of small dictionaries (4-8 keys each) through the validi bridge. The bridge builds its dictionaries/maps with no capacity hint, so each one starts at the minimum size and grows as it fills — at 4-8 keys that's multiple resize boundaries crossed per item, each one a rehash/realloc plus a free of the smaller backing buffer. Multiply by the number of items and we're churning a lotttt of transient memory for dictionaries whose final size we already know up front.

The array bridging cases elsewhere in this file already do pre-allocation, so it makes sense to mirror that, I think.

• JS -> ObjC (NSObjectFromValue): build the Map and TypedObject NSMutableDictionary with dictionaryWithCapacity: (map size / property count), matching the Array case right below it.
• ObjC -> JS (ValueFromNSDictionary): reserve(dict.count) on the ValueMap before the insert loop, matching the typed and array paths.

There's also obviously some bigger optimization options but this one stood out as a good "starter bug" candidate.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Documentation improvement
• Performance optimization
• Test improvement
• Other (please describe)

Testing

• Tests pass locally (bazel test //...)
• Added/updated tests for changes (if applicable)
• Tested on multiple platforms (iOS/Android/Web/macOS as applicable)
• Manual testing performed (describe below)

Testing Details

Covered by the existing test suite (valdi/test/ios/SCValdiMarshallerTests.m) — testCanMarshallMap / testCanUnmarshallMap round-trip an untyped NSDictionary through both changed functions (ValueFromNSDictionary and NSObjectFromValue's map case). No new tests since only the initial capacity changes, not the contents or ordering.

Ran bazel test //... locally: 63/72 targets pass, including everything this change compiles into (//valdi:valdi_ios_objc_test, valdi_ios_swift_test, valdi_macos_objc_test). The 7 that fail (e.g. //valdi:test_java, //libs/dummy:test) fail identically on a clean main checkout, so that's pre-existing local breakage rather than anything from this change.

Checklist

• Code follows project style guidelines
• Documentation updated (if needed)
• No breaking changes (or documented in description)
• Commit messages follow conventional format
• No secrets, API keys, or internal URLs included

Related Issues

Additional Context

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[clholgat]

This looks good, thanks for PR!