Add lazy slice array view

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Motivation

After #814, sjsonnet has lazy array views for large stdlib arrays, but array slicing and std.remove / std.removeAt still had paths that eagerly allocated new Array[Eval] backing arrays.

This PR adds a focused slice view so large slices avoid copying unless fully materialized, while keeping JVM/Scala Native behavior conservative.

Constraints:

• keep Jsonnet indexed laziness: length, eval(i), and value(i) stay O(1)
• do not force elements while slicing
• keep small slices eager to avoid retaining large sources
• prevent deep concat trees
• keep hot paths JIT/GC friendly

Modification

Add a lazy SliceArr view and route Arr.sliced(...) through it when the slice is large enough or the source is already compact/view-backed.

Changed behavior:

• array slicing can return SliceArr instead of eagerly copying Array[Eval]
• std.remove and std.removeAt reuse slice + concat views
• compact sources (RangeArr, ByteArr, lazy indexed arrays, repeat, slice) can slice as O(1) views
• flat/reversed/concat arrays only use a view when the slice is large enough to justify source retention
• concat still caps tree depth by flattening when either side is already a concat view

Result

Verification passed:

• ./mill --no-server 'sjsonnet.jvm[3.3.7].compile'
• ./mill --no-server 'sjsonnet.jvm[2.13.18].compile'
• ./mill --no-server 'sjsonnet.jvm[2.12.21].compile'
• ./mill --no-server 'sjsonnet.jvm[3.3.7].test.testOnly' sjsonnet.ValArrayViewTests sjsonnet.Std0150FunctionsTests
• ./mill --no-server 'sjsonnet.jvm[3.3.7].test'
• ./mill --no-server 'sjsonnet.jvm[3.3.7].checkFormat'
• ./mill --no-server bench.checkFormat
• ./mill --no-server 'sjsonnet.native[3.3.7].nativeLink'

JMH, full JVM regression sweep, compared with upstream/master (lower is better, notable result only):

• Baseline: upstream/master at b4c667d5
• PR head: 3bde215b
• Command: ./mill --no-server bench.runRegressions
• Full sweep covered 45 regression inputs; non-target movement was noise-level, so only the targeted slice/remove case is listed.

Benchmark	Before	After	Result
lazy_array_slice_remove	5.805 ms/op	1.088 ms/op	5.34x faster

Scala Native hyperfine, full regression-input sweep, compared with upstream/master (lower is better, notable result only):

• Binaries: ./mill --no-server show 'sjsonnet.native[3.3.7].nativeLink'
• Command shape: hyperfine --warmup 5 --min-runs 20 -N --output=null ...
• Full sweep covered the same 45 regression inputs; bench.07 was run with ulimit -s 65520 for both sides because the native binary needs a larger process stack for that input.

Benchmark	Before	After	Result
lazy_array_slice_remove	13.2 +/- 0.4 ms	5.89 +/- 0.32 ms	2.24x faster

External performance diff, against jrsonnet built from source at 80cd36a with cargo build --release -p jrsonnet (jrsonnet 0.5.0-pre98):

Benchmark	sjsonnet Scala Native (#821)	source-built jrsonnet	Result
lazy_array_slice_remove	5.8 ms +/- 0.2 ms	7.0 ms +/- 0.2 ms	sjsonnet 1.21 +/- 0.05x faster

JIT / GC review:

• SliceArr preserves indexed laziness: eval(i) returns an Eval; value(i) forces only the requested element.
• Materializing a slice releases the source reference, so long-lived fully-consumed slices do not keep the original array alive.
• Large slices avoid allocating and copying Array[Eval]; small slices still copy to avoid source-retention overhead.
• std.remove / std.removeAt reuse slice and concat views, avoiding large intermediate arrays.
• Concat depth remains bounded.

Rollback boundary:

• This PR only changes slice/remove array representation.
• If a retained-source workload regresses, the slice threshold is the rollback lever without changing the public API.

References

• Builds on Add lazy array views for large stdlib arrays #814 lazy array architecture.
• Follow-up stack: Add array eval copy API #822 adds shared Arr.copyEvalTo; Presize array copy consumers #823 presizes selected copy consumers.