Populate artifacts and add a Hermes bytecode inspector via hbcdump

[pavlof01]

Context

Supersedes #6, which was closed as completed although apps/engine-hermes/src/server.ts still runs a single hermes -dump-bytecode invocation.

Two things are being left on the table:

• Artifact is part of the API contract (apps/api/src/types.ts), documented in README, and every engine returns artifacts: []. There is no way to download anything.
• engines/dockerfiles/Dockerfile.hermes builds and installs hbcdump (lines 31, 52, 57) and nothing ever calls it. hbcdump reads a compiled .hbc bundle and prints the function table, the string table, literal buffers, and a pretty disassembly — an inspector nothing on the web offers.

Proposal

• Compile the snippet with hermesc -emit-binary into the temp dir, return the .hbc as a real Artifact (kind: "bytecode", base64), and let the UI offer it as a download.
• Add a second Hermes mode driven by hbcdump: function table, string table, literal buffers, -pretty-disassemble.
• Surface the mode as a tab in the Hermes output pane.
• Keep artifacts optional and empty for the other engines, so the contract stays as documented.

Acceptance criteria

• A Hermes run returns a non-empty artifacts array and the file downloads and loads in hbcdump locally.
• The inspector tab renders the function and string tables for a small snippet.
• Output stays inside MAX_OUTPUT_BYTES; oversized bundles are reported, not truncated silently.