fix: preserve byte renderer fallback context

[He-Pin]

Motivation:
ByteRenderer's direct materialization path falls back to the generic stackless materializer for very deep arrays/objects. That fallback previously started via Materializer.apply0, which creates a fresh MaterializeContext and drops the objects already being materialized on the active ByteRenderer path.

Key Design Decision:
Reuse the active MaterializeContext when switching from ByteRenderer's direct recursive path to stackless materialization. This keeps visitedObjects, sort behavior, assertion logic, and materialization limits consistent across the fallback boundary.

Modification:

• Change deep array/object ByteRenderer fallback to call Materializer.materializeStackless(v, this, ctx).
• Widen materializeStackless to private[sjsonnet] so ByteRenderer can reuse it without exposing it publicly.
• Add a regression test that proves the old path duplicated partial output before detecting a recursive object, while the fixed path preserves the outer cycle context.

Benchmark Results:
N/A — correctness-only split from render optimization review material. No performance claim is made for this PR.

Analysis:
The old fallback still eventually reported the same recursive materialization error, but it did extra work with a fresh context first. The regression test uses a recursive object with a large string field and a low fallback threshold: the fixed path flushes one copy of the large field before detecting the cycle, while the old path re-rendered the field again before failing.

Validation:

• ./mill --no-server --ticker false --color false -j 1 'sjsonnet.jvm[3.3.7]'.test.testOnly sjsonnet.RendererTests passed.
• ./mill --no-server --ticker false --color false -j 1 'sjsonnet.jvm[3.3.7]'.test passed.
• ./mill --no-server --ticker false --color false -j 1 __.test passed.
• ./mill --no-server --ticker false --color false -j 1 __.checkFormat passed.

References:

• Split from render optimization review/source material in perf: comprehensive Scala Native render pipeline optimization #776.
• Local commit: 7651bb7.

Result:
ByteRenderer fallback now preserves recursive-object detection context across the direct-to-stackless transition without changing external APIs or successful rendering semantics.