test: apply QA sweep fixes across host and sdk tests

[mfw78]

What

Sweep fixes applied:

• Env flags read by value, not presence: VIDERE_SKIP_MISSING_WASMS/CI now treat empty/0/false as unset via flag_enabled + env_flag in crates/videre-host/tests/common/mod.rs, value semantics pinned by a new wasm_helper.rs test (commit d621858).
• Cross-copy whitespace-predicate pin: exhaustive test padded_agrees_with_str_trim_over_every_code_point in crates/videre-sdk/src/client.rs walks all 1,112,064 code points at both ends against the str::trim oracle, ~0.85s under nextest (commit 3f89e6a).
• zero_leak.rs dedup: reuses common::make_wasmtime_engine() and common::block(1), drops the byte-identical inline wasmtime config and Block literal plus the now-unused nexum bindings import (commit 7381f18).
• Banner-hook exemption tightened (3 lens dupes consolidated): .claude/hooks/content-lint.sh now exempts only a true markdown table separator row (pipes/dashes/colons/spaces after the comment marker); pipe-free runs, =/*/_ runs, and dash runs beside other text count as banners again; probe-tested 7 banner shapes match and 7 table/clean shapes exempt, hook smoke-tested end-to-end (blocks banner, passes table row), zero hits repo-wide with the new pattern (commit 34e27fd).
• Venue-brand neutralization: the bounded-fetch spy transport fixture in crates/videre-sdk/src/transport.rs never dials, so its request URL was decoration; swapped the one remaining real venue-brand string (api.cow.fi) for the reserved api.venue.example (commit 9a579e7).

Why

Part of #27.

Testing

All run inside nix develop (rustc 1.94.0, nextest 0.9.127) with RUSTFLAGS=-D warnings on the final tree:

• cargo fmt --all --check PASS (after folding a rustfmt let-else expansion into the sdk test commit; first run failed on exactly that)
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings PASS
• cargo test --doc --workspace --all-features --locked PASS
• RUSTDOCFLAGS=-D warnings cargo doc --no-deps --workspace --locked PASS
• just build-modules PASS, then cargo nextest run --workspace --all-features --no-fail-fast --locked PASS: 219/219 passed, 1 skipped (the pre-existing #[ignore]d videre-test regenerate_reference_fixtures, which passes when run with --run-ignored)
• Both new tests confirmed executed and passing
• Venue-brand rg (cowswap|cow.fi|cow protocol|cowprotocol|gpv2|composable|uniswap|1inch|balancer|sushiswap, case-insensitive) over videre-sdk/videre-host/videre-macros: zero hits (the repo's neutral placeholder id cow and std::borrow::Cow are baseline convention, not brand strings)
• Banner-pattern rg over added lines of origin/refactor/host-wall-clock-seam..HEAD (.rs/.md): zero hits
• Commit-message sweep of that same range for co-authored-by: clean

Stacked: merge bottom-up; full CI runs after retarget.

AI Assistance: Claude Code (Sonnet) implemented and tested these sweep fixes.