fix(runtime): keep preload shims transparent on failure

[yunwei37]

Summary

• make syscall-server, agent, and transformer failures fall back without terminating the host
• keep preload logging off stdout/stderr by default and make logger sink failures silent
• preserve entry errno, relative openat semantics, and single-call mmap fallback behavior
• restore transformed text permissions on every exit path and keep partially initialized hooks delegated to the original syscall
• clean up partially initialized agent context, PID, and shared-memory state without racing the IPC thread
• route runtime trace output through the configured logger and remove teardown stderr writes
• record the preload safety contract in the root maintenance instructions, with AGENTS.md linked to the existing instruction file and no root docs/ directory
• add failure-injection regression coverage for shared-memory exhaustion, logger write failure, mmap/openat fallback, missing agent state, transformer cleanup, host exit status, and host stdio preservation

Validation

• cmake --build build --target bpftime-agent bpftime-syscall-server bpftime_runtime_tests bpftime-shm-allocation-test-helper -j2
• build/runtime/unit-test/bpftime_runtime_tests '[allocation],[preload]' (7 test cases, 42 assertions)
• 74 runtime test cases that do not require the LLVM VM backend (8,575,172 assertions)
• local example/minimal uprobe end-to-end run with the repository test driver: server startup, agent attach, and target_func called. assertion passed
• local privileged example/minimal syscall-transformer end-to-end run with the repository test driver: text rewrite, agent install, and syscall write called. assertion passed
• integrated examples workflow on the reviewed code head: 120/120 jobs successful
• independent review of the final code diff found no blocking issues

The local build used the ubpf backend, so the eight attach/tail-call cases that require the LLVM VM backend were excluded from the broader local runtime-test run. Console logging was explicitly selected only for the two example assertions.

Fixes #605

[yunwei37]

Addressed the final review findings in 796c895:

• removed the duplicate original mmap call and preserved relative openat fallback semantics;
• captured caller errno at every wrapper boundary and restored it on successful fallback;
• made spdlog sink write/flush failures silent;
• added RAII cleanup for text permissions and Capstone state;
• added deterministic preload regressions for /dev/full, exact-once mmap, relative openat, errno preservation, and RX -> RWX -> RX transformer fault cleanup.

The focused suite now passes 7 test cases / 42 assertions, and the broader non-LLVM run passes 74 test cases / 8,575,172 assertions. Final independent review found no blocking issues. I will continue monitoring checks and review feedback.