docs: steer low-memory installs to prebuilt binaries

[dubadub]

Addresses the install side of #366.

We already publish aarch64-unknown-linux-musl and arm-unknown-linux-musleabihf binaries in release.yaml — but the README presents cargo install cookcli as a peer option with no caveats, so SBC users reach for it and then spend an hour compiling ~400 crates only to run out of RAM.

Changes to the Installation section:

• cargo install cookcli → cargo install cookcli --locked, with a note on why.
• Say plainly that installing via cargo compiles from source and needs a few GB, and point memory-constrained users at the prebuilt binaries we already ship.
• A collapsed section documenting the Armbian trap: /tmp is zram-backed (sized RAM/2) and cargo install puts its target dir there, which is what produces the confusing failed to load bitcode of module ... failure. Gives the CARGO_TARGET_DIR and CARGO_PROFILE_RELEASE_LTO=false escape hatches.

Pairs with #367, which fixes the underlying memory cliff by moving off fat LTO.

Refs #366

[claude]

Review

Reviewed the README diff (docs-only change, no Rust code touched, so cargo fmt/clippy/test don't apply here).

Overall: solid, well-scoped documentation fix. Verified the technical claims:

• aarch64-unknown-linux-musl and arm-unknown-linux-musleabihf are indeed published in release.yaml, so the "prebuilt binaries" pointer is accurate.
• CARGO_PROFILE_RELEASE_LTO is a real Cargo profile override env var and works regardless of whether Cargo.toml's [profile.release] currently has lto = true (as on main today) or lto = "thin" (as proposed in fix(build): use thin LTO to cut peak build memory #367) — so this doc doesn't depend on fix(build): use thin LTO to cut peak build memory #367 landing first or in any particular order.
• Link casing (cooklang/CookCLI) is consistent with the rest of the README.
• <details>/<summary> block is well-formed with blank lines around the fenced code blocks, so it should render correctly on GitHub.

Minor nits (non-blocking):

• "optimisation" (line 185) is the only British-spelling word in the file; the rest of the README uses American English ("organize", "customize", etc. elsewhere in the repo). Consider "optimization" for consistency.
• The zram-causes-OOM explanation is a specific technical claim about Armbian's /tmp mount; worth double-checking against current Armbian defaults before merging, since it's stated as unqualified fact rather than "typically."

No security, performance, or test-coverage concerns — this is a README-only change with no executable code paths affected.