feat: add pipeline concurrency throttling and HPC build docs

Introduces a counting semaphore-based throttling mechanism to limit concurrent file I/O and pipeline processing. Replaces custom path wrappers with standardized `Throttled` types across `obikmer` and `obikpartitionner`, ensuring RAII-based resource cleanup and explicit backpressure. Additionally, documents how to redirect Cargo build artifacts to local scratch storage on HPC filesystems to prevent compilation slowdowns.

docmd/installation.md

@@ -53,6 +53,22 @@ cargo build --release
 
 The compiled binary is at `target/release/obikmer`.
 
+### Building on HPC clusters (network filesystems)
+
+HPC home directories are typically on a network filesystem (Lustre, NFS) optimised for large sequential reads — not for the thousands of small file operations that Cargo generates during compilation. Building directly on such a filesystem can be extremely slow (0.1% CPU utilisation, tens of minutes for what should take seconds).
+
+**Always redirect the build directory to a local scratch disk:**
+
+```bash
+CARGO_TARGET_DIR=/scratch/local/$USER/cargo-target cargo build --release
+```
+
+Adapt the path to the local scratch available on your cluster (`/var/tmp`, `/tmp`, `/scratch/local`, etc.). Once built, copy the binary to a permanent location:
+
+```bash
+cp /scratch/local/$USER/cargo-target/release/obikmer ~/bin/
+```
+
 ## NUMA support
 
 NUMA-aware thread placement is active automatically on multi-socket Linux machines (detected at runtime via hwloc). No special build flag is required — the detection is built in and falls back gracefully to the single-pool adaptive strategy on: