Enforce CREATE IF NOT EXISTS across all app DDLs (closes #34)

[gangtao]

Summary

Closes #34. Makes every CREATE in apps/*/ddl/ idempotent so app upgrades (which re-run the DDL against an existing database) don't fail with already exists.

• alpha-101, market-data: converted 43 CREATE OR REPLACE VIEW → CREATE VIEW IF NOT EXISTS. OR REPLACE drops and recreates the view, breaking dependent MVs / dashboards during the recreate window; IF NOT EXISTS preserves the view as a stable dependency.
• cisco-asa-ddos: added IF NOT EXISTS to the 3 CREATE RANDOM STREAM files that were missing it.
• skill/SKILL.md: turned the one-line "use IF NOT EXISTS" tip into a proper Idempotency section with the rule, the udf exception (CREATE OR REPLACE FUNCTION — UDF bodies are what change on upgrade), and the two parser-level non-supporters (SYSTEM INSTALL PYTHON PACKAGE, and db.-qualified UDF / format schema names). Added a Common Mistakes entry for the upgrade-failure symptom.
• alpha-101: bumped to 1.0.1, added an SVG icon (Greek α on the standard pink→purple gradient), and refreshed the description to reflect all 10 implemented alphas (Machine Learning Feature Pipeline App #1, Feature/1 machine learning feature pipeline app #2, Hacker New Demo #3, add hacker news demo #4, Cisco ASA Log DDoS Demo #6, market data product filter for dashboard #9, update icons #12, update #22, ocsf: add OCSF security event analytics demo app #41, #54) instead of just two.
• cisco-asa-ddos: bumped to 1.0.2.
• market-data: bumped to 1.1.1.

Validation

Tested CREATE … IF NOT EXISTS on the running Timeplus (proton 3.3.1-rc.9) for every framework resource type (stream, external_stream, mutable_stream, materialized_view, view, external_table, udf, task, alert, input, dictionary, format_schema, named_collection), plus random_stream and the access-control resources (user, role, row_policy, quota, settings_profile). All accept IF NOT EXISTS and are idempotent on rerun. The only outlier in the framework's surface is SYSTEM INSTALL PYTHON PACKAGE, which the parser rejects — but it's idempotent on re-run, and the project convention is to declare Python deps in manifest.yaml (python_packages:), not as DDL.

Test plan

• make build APP=alpha-101 && make build APP=market-data && make build APP=cisco-asa-ddos — all three packages build clean (verified locally).
• Install each app fresh against a clean Timeplus, then re-install the same .tpapp over the top — second install should succeed with no already exists errors.
• Confirm the alpha-101 icon renders correctly in the Timeplus UI app cards.