fix: a relative current_dir runs a relative program, and inspect's help says what it does

[vyncint]

A relative TerminalBuilder::current_dir, and so termlens inspect --cwd ./examples ./myapp, never worked with a relative program path. The PTY
layer joined the directory onto the program, ./examples/./myapp, and the
child then entered the directory before exec, so the path was looked up a
second time from inside it and did not exist. The exec failure could not
even be reported: the PTY layer's pre-exec hook closes every inherited
descriptor, std's error pipe included, so the child died of a runtime
abort, fatal runtime error: assertion failed: output.write(&bytes) .is_ok(), which inspect showed as exited: killed by signal: Aborted.
0.11.4 behaves the same. spawn now makes a relative directory absolute
against the test process's own first, as std::process::Command treats
one, and a relative program then resolves inside it as documented.

Tests, each checked to fail without the fix:

• process.rs spawns ./emit from a relative current_dir. The scratch
  directory sits deeper than the test's own working directory, because a
  directory as deep as it, like target/debug beside crates/termlens, maps
  onto itself when the path is applied twice and hid the bug in a first
  version of this test. It now asserts that before trusting a pass.
• inspect.rs runs the example and termlens inspect with
  --cwd inspect-relative ./echo from the directory above.

The help of termlens inspect and examples/inspect.rs, which mirror each
other, now says what three behaviours are, all checked by running them:

• the silence window starts only once the program has drawn something,
  so a program that only clears the screen runs to the deadline;
• the child also gets TERM=xterm-256color and SHELL=/bin/sh, and
  --inherit-env keeps the caller's environment but for TERM;
• --cwd's argument is DIR, not PATH beside the PATH variable, in the help
  and in its diagnostic (pinned in both test suites), and a relative
  program path is found inside it.

The CLI README's --cwd ./examples ./myapp example is the broken case;
it and its paragraph now say where the program is found, as does the
skill's line. Found by a documentation review.