feat(env): format-preserving byte-level YAML splice fast path

[fentas]

Follow-up from PR #126 review: every successful merge sync used to produce a noisy git diff because `spliceYAMLStructural` round-tripped the whole document through `yaml.Marshal`, which normalizes whitespace, quoting, and field ordering even for keys the splice didn't touch.

What's new

`spliceYAMLByteLevel` is a new fast path that walks `local` byte-by-byte and only re-emits the in-scope keys:

1. Parse local for `yaml.v3` Node Line/Column metadata.
2. Compute byte ranges for every top-level key.
3. Walk local:
   • out-of-scope range → copy verbatim
   • in-scope, in merged → emit `yaml.Marshal` of just that one key:value pair
   • in-scope, not in merged → drop (deletion)
4. Append additions (merged keys not in local) at the end.

The output is byte-identical for every unchanged byte. Only the in-scope keys go through the yaml.v3 emitter.

Dispatch order in `spliceYAML`

1. Byte-level splice (best fidelity, NEW)
2. Structural splice (fallback for shapes byte-level can't handle — empty doc, non-mapping root, exotic Line metadata)
3. Text splice (conflict path, unchanged)

Test

`TestSpliceYAMLByteLevel_PreservesOutOfScopeBytesVerbatim` uses a local file with non-default formatting:

```yaml

Top of file

binaries:
kubectl: {} # 4-space indent
kustomize: {}

envs:
"github.com/keep/me": # an inline comment
files:
"a.yaml": "docs/a.yaml" # double-quoted keys
# trailing comment inside envs
```

After splicing in a new `tilt:` binary, all of the formatting choices above (4-space indent, quoted keys, inline comment, trailing comment, header comment) survive byte-for-byte. Before this PR, every one of them would be reformatted by yaml.v3.

This is the format-preserving emitter mentally tracked from the #126 review loop.

🤖 Generated with Claude Code

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Pull request overview

Adds a new YAML splice fast path that preserves untouched bytes verbatim during select + merge env syncs, reducing noisy diffs caused by full-document yaml.v3 re-emission.

Changes:

• Introduces spliceYAMLByteLevel and updates spliceYAML dispatch order to prefer byte-level splicing, then structural, then text/conflict splice.
• Adds a unit test intended to validate that out-of-scope YAML formatting is preserved when splicing in-scope changes.

Reviewed changes

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File	Description
pkg/env/splice.go	Adds byte-level YAML splice strategy and adjusts dispatch to prioritize format-preserving behavior.
pkg/env/splice_test.go	Adds a new test case for byte-level splice preservation behavior.

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Comments suppressed due to low confidence (1)

pkg/env/splice.go:36

• The spliceSelectedScope doc comment still describes only two strategies (structural + text), but spliceYAML now has a byte-level fast path that is the default for successful YAML merges. Please update this top-level comment to reflect the three-path dispatch so callers understand when output is byte-preserving vs re-encoded.

// Two paths:
//
//   - Structural splice (fast path): when `merged` parses as valid YAML,
//     rewrite the local Node tree in place (replace scoped key values,
//     append new scoped keys, remove vanished scoped keys). Out-of-scope
//     comments and layout are preserved because their Nodes are untouched.
//     Note: the yaml.v3 encoder re-emits the whole document, so the output won't
//     be byte-identical to the input even for unchanged keys — this is a
//     limitation of yaml.v3 that the structural splice cannot work around
//     here. A format-preserving emitter is tracked as a separate
//     follow-up.

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