Stream large staged files into Playground without whole-file base64 allocation

[chubes4]

Problem

WP Codebox staged-file materialization base64-encodes and submits an entire file through one Playground PHP evaluation. A valid 65.7 MB SSI artifact therefore exhausts the fixed 256 MB PHP limit before any recipe workflow step executes.

This blocks the canonical FSE Pilot Build Theme Figma-to-WordPress acceptance fixture. The isolated single-fixture recovery fails identically.

Deterministic evidence

• FSE artifact.json: 65,696,394 bytes.
• Failed allocation: 87,595,192 bytes.
• Exact relationship: 65,696,394 * 4 / 3 = 87,595,192, proving whole-file base64 expansion.
• Staged FSE source: 49,268,534 bytes.
• Fisiostetic control artifact: 11,379,992 bytes and succeeds under the same runtime.
• Failure: Allowed memory size of 268435456 bytes exhausted (tried to allocate 87595192 bytes) in /internal/eval.php on line 7.
• Both batch and isolated recovery have executions: []; no SSI import or WordPress operation starts.

Reproduction

From current Automattic/static-site-importer and a Blocks Engine checkout containing the FSE transform:

node tools/run-fig-fixture-e2e.mjs \
  --blocks-engine /path/to/blocks-engine \
  --blocks-engine-php-transformer-path /path/to/blocks-engine \
  --fixture "/path/to/🛠️ FSE Pilot Build Theme.fig" \
  --output-directory /tmp/fse-current-e2e \
  --expected-fixture-count 1 --batch-size 1 --concurrency 1 \
  --min-native-rate 1 --run

The generated replay command in ssi-matrix/summary.json reproduces the same failure.

Owning path

packages/runtime-playground/src/mount-materialization.ts reads staged sources fully, creates whole-file base64/string copies, and sends them through Playground writeFile / PHP fallback materialization.

Expected behavior

Materialize large staged files with bounded peak memory, using chunked truncate/append writes or an equivalent streaming primitive. Preserve exact bytes and existing mount semantics. Add a regression above the current practical 256 MB-runtime threshold that proves:

• byte-identical materialization;
• bounded per-write payload size;
• no whole-file base64 allocation;
• compatibility for text and binary staged files;
• actionable bounded diagnostics on failure.

Raising PHP memory to 512 MB can validate the diagnosis but is not the durable repair.