[off-CPU stack 2/3] Add dynamic ProbeContext runtime foundation

[dalehamel]

Stack

1. #73 → #50
2. This PR — minimal Probe runtime foundation
3. #75 — duration-filtered off-CPU probe + task-state labels

What

Port the generic Probe / ProbeContext loading API onto the upstream-synced LuaJIT omnibus base, plus only the reusable runtime capabilities explicitly required by off_cpu_time:

• dynamic origin registration before attachment;
• isolated .rodata / .rodata.var maps per probe instance;
• removal of probe-private maps/programs/variables from the main collection via the <configured name>_* convention;
• shared latency config and owned-resource lifecycle helpers;
• typed dynamic metrics subscriptions with aggregation, panic isolation, and explicit final counter flushing;
• label-aware sample identity so equal stacks with different per-sample labels do not merge.

It includes the generic demo probe from Shopify #58, but intentionally excludes every Hack Days I/O, async, process-observer, userspace-target, lock-wait, heap, and off-CPU implementation.

Why these additions to #58

The original #58 API was subsequently hardened by the synchronous probes and process-aware probes. off_cpu_time explicitly depends on isolated RODATA, private-resource unloading, probeutil, metric registration, and label-aware sample keys; extracting these generic pieces avoids depending on #53/#63/#65 wholesale.

Omnibus adaptation

PR #50 had one LuaJIT ProgLoaderHelper literal using the old private field names. This layer ports it to the exported API (ProgID, Name, Enable) without changing behavior.

Validation

• go build, go test, and go vet across probes, tracer, reporter, traceutil, and controller.
• New Probe API lifecycle/metrics tests use existing counter/gauge IDs and have no userspace-target dependency.
• Generic and probeutil tests green.

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