refactor(core): extract no_std + alloc surface from ssg-core

[sebastienrousseau]

Strategy. crates/ssg-core/ is std-only despite shipping a wasm32 build. Splitting ssg-core's parser surface into no_std + alloc unlocks embedded/IoT documentation builds and aligns with the DevRel angle for the "edge embedded firmware" narrative.

Files

• Edit: crates/ssg-core/Cargo.toml:

  [features]
  default = ["std"]
  std = []

• Edit: crates/ssg-core/src/lib.rs: #![cfg_attr(not(feature = "std"), no_std)] extern crate alloc;
• Edit: hot paths under crates/ssg-core/src/ — replace String allocations on parse paths with &str borrows where possible; gate std::time, std::env, std::fs use behind cfg(feature = "std").

Acceptance criteria

• cargo build -p ssg-core --no-default-features --target thumbv7em-none-eabi succeeds.
• cargo build -p ssg-core --no-default-features --target wasm32-unknown-unknown succeeds (slimmer than today's wasm32 build).
• New CI matrix entry in wasm.yml: build ssg-core for both wasm32-unknown-unknown and thumbv7em-none-eabi.

Marketing. Enables the DevRel angle: "the only static-site generator whose parser can run on a Cortex-M4 firmware doc-build."